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XB

The XB designation was a prefix in the United States military aircraft naming system, specifically denoting experimental bomber aircraft developed for the United States Army Air Forces (USAAF) and later the United States Air Force (USAF).^[1] Introduced in the interwar period as part of early type-based conventions, XB signified prototype bombers designed to test advanced technologies for strategic bombing roles, often featuring innovative designs in range, speed, and payload capacity.^[2] This system evolved from the 1924 "Project Number" designations to more descriptive letter-number formats by the 1930s, with XB used for single experimental models preceding service types like the B-17 or B-29.^[1] Prominent examples include the Douglas XB-19, the largest U.S. bomber of World War II era, and the North American XB-70 Valkyrie, a Cold War-era Mach 3+ prototype that influenced high-speed aviation.^[3]^[4] The 1962 Tri-Service aircraft designation system unified naming across branches, retaining the "X" prefix for experimental variants of bombers (e.g., XB-70), though production bombers shifted to simpler B-numbers. XB projects contributed significantly to U.S. aerial warfare capabilities, from long-range heavy bombers in the pre-World War II era to jet-powered strategic platforms during the Cold War, paving the way for modern designations.^[1]

Designation System

Meaning and Conventions

The XB designation refers to an experimental bomber within the United States Army Air Corps and Air Force aircraft naming system, which was established in 1924 and used until 1962. In this framework, the "X" prefix signifies an experimental prototype intended for testing and development, while the "B" indicates the bomber mission category, encompassing aircraft designed to evaluate strategic or tactical bombing capabilities.^[5]^[6] Numbering for XB aircraft follows a sequential integer system within the bomber series, beginning with XB-1 in 1924 and continuing up to XB-70, without regard to manufacturer-specific models. Experimental prototypes carried the XB designation prior to any potential transition to service-test models designated YB or production variants as B.^[5]^[7]^[6] The XB prefix is distinct from related experimental designations in the same system, such as XA for experimental attack aircraft focused on ground support roles, XP for experimental pursuit or fighter aircraft emphasizing air-to-air combat, and XH for experimental helicopters developed for rotary-wing applications. Unlike these, XB is reserved exclusively for bomber prototypes that test heavy payload delivery, range, and bombing accuracy without operational deployment in mind.^[5]^[7] Key conventions for XB aircraft emphasize limited production, with typically only one or a few units constructed solely for evaluation purposes, ensuring resources are allocated to proof-of-concept rather than mass manufacturing. These prototypes are not intended for combat or routine service use, serving instead as platforms to validate aerodynamic, structural, and armament innovations. Redesignations are possible if testing proves successful, such as converting an XB to a standard B designation upon adoption for production. This system persisted until 1962, when it was superseded by the unified Tri-Service aircraft designation framework.^[5]^[6]^[7]

Historical Context and Evolution

The origins of the XB designation trace back to the United States Army Air Service's aircraft numbering system established in September 1919, which used numerical types and letter codes to categorize aircraft by function, with subsequent expansions to 23 types by 1924.^[5] This system underwent significant revision in 1924 to address inconsistencies, introducing a more unified structure with mission-based letters (such as "B" for bomber) and status prefixes like "X" for experimental prototypes, thereby formalizing the XB prefix for experimental bombers.^[8] The first aircraft to receive the XB designation was the Huff-Daland XB-1 (later redesignated Keystone XB-1), a twin-engine heavy bomber prototype that flew in 1927, marking the beginning of a sequential numbering system for bombers that emphasized testing innovative designs for potential service adoption.^[9] During the 1930s, the XB system evolved within the newly formed United States Army Air Corps, established by the Air Corps Act of 1926, which renamed the Air Service, mandated pilot-led units, and authorized expansion including dedicated experimental programs to advance aviation technology.^[10] General Billy Mitchell's advocacy for airpower independence and strategic bombing played a key role in this period, influencing doctrine at the Air Corps Tactical School to prioritize long-range bombers capable of striking vital enemy centers, as demonstrated in his 1921 trials that proved aerial bombardment's potential against naval targets.^[10] The Act also spurred a shift to all-metal construction post-1926, enabling more durable and aerodynamically efficient designs like the Martin B-10 (1934) and Boeing B-17 (1935), which tested concepts under the XB framework before production.^[10] This era saw XB projects focus on enhancing range and payload, aligning with Mitchell's vision of air forces as a dominant offensive arm independent of ground support.^[5] World War II accelerated XB development through an emphasis on rapid prototyping to meet urgent operational needs, resulting in over 20 XB designations for bombers, including the XB-15, XB-19, XB-29, XB-32, XB-35, XB-36, XB-40, XB-42, XB-43, and XB-45, which tested heavy, medium, and specialized configurations amid the push for mass production.^[8] The 1947 National Security Act transferred Army Air Forces functions to the newly independent United States Air Force, preserving the XB system without immediate changes to support ongoing experimental efforts in the postwar environment.^[11] However, post-1948 shifts toward the jet age led to cancellations of several piston-engine XB projects, such as the Martin XB-48 (canceled in 1948) and Boeing XB-55 (canceled in 1949), as resources pivoted to turbojet designs like the XB-45 and XB-47 to achieve higher speeds and altitudes. The XB designation persisted until the 1962 Tri-Service unification, which standardized designations across Army, Navy, and Air Force services under a mission-based system (e.g., "B" for bomber), effectively ending the pre-1962 sequential numbering for new types while retaining the "X" prefix for experimental variants like the North American XB-70.^[7] Although the unified system replaced many legacy sequences—shifting to function-led identifiers such as the B-1 Lancer—the "XB" format continued informally in non-military contexts, as seen with Boom Supersonic's XB-1 supersonic demonstrator named in 2019 to evoke historical experimental traditions. The XB-1 achieved its maiden flight on March 22, 2024, and broke the sound barrier on January 28, 2025 (Mach 1.1), followed by a second supersonic flight on February 10, 2025 (Mach 1.122), validating technologies for sustainable supersonic flight.^[7]^[12]

Key Periods of Development

Pre-World War II Era (1924–1941)

The pre-World War II era marked the inception of the XB designation system for experimental bombers within the United States Army Air Corps, focusing on transitioning from biplane designs to more advanced monoplane configurations amid fiscal constraints and technological experimentation. Early efforts emphasized biplane prototypes suited for limited roles, such as the Huff-Daland XB-1, which achieved its first flight in 1927 as the inaugural aircraft to receive a "B-" bomber designation. Powered by twin Liberty 12A engines producing 400 horsepower each, the XB-1 featured a 68-foot wingspan and attained a maximum speed of approximately 106 miles per hour, serving primarily as a testbed for heavy bomber concepts but ultimately highlighting the limitations of wood-and-fabric construction in multi-engine applications. Similarly, the Glenn L. Martin Company's XB-3 design from 1927 represented an abandoned early attempt at a twin-engine biplane bomber, influenced by post-World War I requirements for torpedo and bombing capabilities, though it never progressed beyond conceptual stages due to evolving priorities.^[13] By the 1930s, advancements shifted toward all-metal monoplanes and radial engines, driven by Air Corps requests for improved speed and range, though many projects remained conceptual or unbuilt owing to the Great Depression's funding cuts. The Douglas XB-7, redesignated from an incomplete XO-36 observation aircraft, made its first flight in May 1931 and became the first U.S. monoplane to earn a bomber designation, equipped with Pratt & Whitney R-1690-9 radial engines and bomb racks for up to 1,200 pounds of ordnance, achieving a top speed of 177 miles per hour during testing.^[14] The Boeing XB-9, first flown on April 29, 1931, pioneered all-metal monoplane bomber construction with twin Pratt & Whitney R-1860 Hornet radials, a 77-foot wingspan, and a top speed of 186 miles per hour, directly influencing the production YB-9 and marking a conceptual leap from biplanes despite its limited production of just one prototype.^[15] Proposed designs requested by the Air Corps but never constructed underscored the era's emphasis on theoretical multi-engine testing over full development. Key projects in the late 1930s exemplified ambitious "super bomber" efforts to achieve transoceanic range, though persistent underpowering and budget isolationism under the 1920s Washington Naval Treaty mindset hampered progress. The Boeing XB-15, contracted in 1934 and first flown on October 15, 1937, featured a 149-foot wingspan, four Pratt & Whitney R-1830 Twin Wasp engines, and demonstrated exceptional endurance with a 5,130-mile closed-circuit flight in 1938, establishing range benchmarks while testing metal airframe durability for future designs.^[16] The Douglas XB-19, initiated under a 1935 Air Corps specification for a long-range heavy bomber, was the largest prewar U.S. aircraft with a 212-foot wingspan and four Allison V-1710 liquid-cooled engines, achieving its maiden flight on June 27, 1941, after years of development that pioneered tricycle landing gear and wing-tunnel systems for crew movement.^[17] Approximately 10 XB designations were issued during this period, with most remaining unproduced prototypes or concepts due to economic pressures and a doctrinal focus on defensive isolationism, prioritizing range over speed in wood-to-metal transitions.^[16]

World War II and Immediate Postwar (1941–1950)

The onset of World War II accelerated the development of experimental bombers under the XB designation, driven by the urgent need for long-range strategic capabilities. The Douglas XB-19, the largest bomber built by the United States before the war, made its first flight on June 27, 1941, from Clover Field in Santa Monica, California.^[18] This massive four-engine piston aircraft, with a wingspan exceeding 212 feet, was underpowered initially but underwent modifications as the XB-19A, incorporating turbo-superchargers to enhance high-altitude performance and achieve a tested ferry range of approximately 7,000 miles, providing valuable data on large-scale bomber design despite its limited operational viability.^[10] Complementing this effort, North American Aviation's XB-28, a medium bomber, first flew on April 26, 1942, featuring a pressurized cabin for high-altitude operations—one of the earliest such implementations in U.S. combat aircraft—and remote-controlled turrets for defense.^[19] However, evolving tactical priorities led to its cancellation in late 1943, after only two prototypes were completed. The transition to jet propulsion marked a pivotal shift during the war's later stages, with the Douglas XB-43 emerging as the first U.S. jet-powered bomber. Ordered in 1943 and derived from the piston-engined XB-42 Mixmaster, the XB-43 replaced its pusher propellers with two General Electric TG-180 (later J35) turbojets, achieving a top speed of about 515 mph in testing. Its maiden flight occurred on May 17, 1946, at Muroc Army Air Field (later Edwards AFB), though early engine reliability issues, including limited flight durations like 100-hour endurance tests, delayed full evaluation.^[20] North American's XB-45 Tornado, contracted in 1944, advanced this trend with four Allison J35 turbojets, attaining a maximum speed of 516 mph at 13,300 feet during its first flight on March 17, 1947, also at Muroc.^[21] This prototype's performance directly influenced the production B-45A, the first U.S. multi-engine jet bomber to enter service, with 142 units built for tactical roles.^[22] Postwar assessments focused on refining jet designs amid budget constraints and rapid technological evolution. The Martin XB-48, initiated in 1944 as a medium bomber competitor to the XB-45, incorporated six J35 turbojets in its straight-wing configuration and introduced innovative tandem landing gear. First flying in 1947, it suffered from unstable handling and poor performance, leading to cancellation in 1948 after limited testing.^[18] Similarly, the Martin XB-51 trijet ground-attack bomber, designed in 1945 for low-level strikes, achieved 518 mph with its three J33 engines and featured variable-incidence wings for short takeoffs, including rocket-assisted tests that simulated vertical-like departures.^[23] Despite promising speed and maneuverability, its complexity and competition from simpler designs like the English Electric Canberra resulted in no production, though it informed future attack aircraft concepts.^[24] Under the War Production Board's oversight, contracts for these XB projects surged in 1944–1945 to meet wartime demands, involving at least eight major experimental efforts that tested piston-to-jet transitions.^[25] Postwar flight tests, including those at Muroc Dry Lake in 1947, validated innovations like cabin pressurization for crew comfort at altitude and remote-control turrets for enhanced gunnery, though challenges such as jet engine reliability persisted across prototypes.^[21] Of these, four advanced to limited YB service variants for further evaluation, bridging the gap to operational jet bombers.^[10]

Notable Aircraft Examples

Cold War Prototypes (1950s–1970s)

During the Cold War, the U.S. Strategic Air Command (SAC) emphasized long-range bombers capable of delivering nuclear weapons in response to Soviet threats, driven by President Eisenhower's "New Look" policy that prioritized massive retaliation through airpower.^[26] This led to the development of experimental XB prototypes focused on speed, altitude, and payload to penetrate enemy defenses, with requirements evolving from subsonic jets to supersonic designs by the mid-1950s.^[27] Several major XB bomber prototypes emerged in this era, influencing later strategic systems amid the rise of intercontinental ballistic missiles (ICBMs).^[28] The Boeing XB-52, rolled out in 1951 and first flown on October 2, 1952, represented a supersonic-era breakthrough as an eight-engine turbojet-powered heavy bomber designed for intercontinental nuclear strikes. Powered by Pratt & Whitney J57 engines, it achieved a top speed of 650 mph (Mach 0.86) and evolved directly into the production B-52 Stratofortress, which entered service in 1955 and remains operational.^[29]^[30] Its tandem cockpit and swept-wing design addressed SAC's need for high-altitude endurance, carrying up to 70,000 pounds of ordnance over 8,800 miles.^[31] Building on jet propulsion advances, the Convair XB-58 Hustler prototype, first flown on November 11, 1956, became the first operational supersonic bomber, reaching Mach 2 with its delta-wing configuration and four General Electric J79 turbojets. Developed to meet SAC's 1950s demand for rapid nuclear delivery evading radar, it featured a sleek fuselage for low drag and pod-mounted fuel/weapons, though high maintenance costs limited production to 116 aircraft before retirement in 1970.^[32]^[33] The North American XB-70 Valkyrie epitomized high-altitude supersonic innovation, with its first prototype (AV-1) flying on September 21, 1964, powered by six General Electric YJ93-GE-3 afterburning turbojets each producing 30,000 pounds of thrust. Designed for Mach 3+ cruises at 73,000 feet using compression lift—where the aircraft rode its own shockwave for efficiency—its 105-foot wingspan and folding wingtips enhanced stability, enabling a 4,288-mile range without refueling.^[4]^[34] Two prototypes were completed between 1964 and 1967, logging 129 flights total to test aerodynamics and materials like titanium for extreme heat.^[4] The B-70 program, initially funded at $800 million, had its operational bomber aspect canceled on March 28, 1961, as ICBMs like the Atlas and Titan rendered high-speed manned bombers vulnerable to improving Soviet surface-to-air missiles, though a joint USAF/NASA research program continued, leading to the XB-70A prototypes. This transition influenced designs like the AGM-28 Hound Dog, tested on B-52s, and later systems that prioritized subsonic penetration over supersonic dash.^[35]^[36] XB-70 testing faced setbacks, including a June 8, 1966, mid-air collision during a formation flight near Edwards Air Force Base, where AV-2 struck an F-104N chase aircraft, killing the F-104 pilot and XB-70 copilot while the XB-70 pilot ejected safely; the wreckage fell into the Mojave Desert.^[34] AV-1 encountered landing gear issues, including a tire blowout and brake fire during its 1964 maiden landing and a 1969 emergency gear extension failure resolved with a makeshift paperclip fix to the hydraulic system.^[37]^[38] The program's data advanced high-speed flight research, contributing to NASA studies on hypersonics despite no production bombers.^[4]

Post-Cold War and Experimental Revival (1980s–Present)

Following the 1962 Tri-Service aircraft designation system overhaul, official XB designations for experimental bombers became rare in U.S. military programs, with no new assignments issued through the 1980s and 1990s as strategic bomber development shifted toward established B-series aircraft and reconnaissance adaptations like the SR-71, which continued the pre-1962 bomber lineage from the XB-70 Valkyrie. The legacy of the XB-70 influenced NASA's subsequent X-plane series, such as the X-15 and later hypersonic demonstrators, but these adopted the unified X- prefix rather than XB-specific bomber notation. In military contexts, programs like the 1990s Advanced Tactical Fighter (ATF) explored advanced designs but assigned YF- designations for fighter prototypes, bypassing any hypothetical XB for bomber variants due to the focus on air superiority roles. The revival of XB-like designations emerged in the private sector during the 2010s, driven by commercial supersonic ambitions amid easing regulatory hurdles. Boom Supersonic's XB-1, a one-third-scale demonstrator nicknamed "Baby Boom," marked a significant milestone as the first privately developed aircraft to adopt the XB nomenclature since the Cold War era. Powered by three Williams FJ33-5A turbofan engines, the XB-1 targeted Mach 1.3 speeds to validate technologies for low-boom supersonic flight, conducting subsonic test flights starting in March 2024 from Mojave Air and Space Port.^[39] Its first supersonic flight (12th overall) occurred on January 28, 2025, reaching Mach 1.122 (750 mph) at 35,290 feet without an audible sonic boom over land, followed by a second supersonic flight (13th and final) on February 10, 2025, reaching Mach 1.18 (772 mph) at 36,514 feet; this concluded the test program, with the aircraft returned to Denver, Colorado, as of February 2025 and no further flights reported as of November 2025, to inform the Overture airliner's design, which aims for 65-80 passenger commercial service by the late 2020s.^[12]^[40]^[41] In parallel, U.S. Department of Defense initiatives in the 2020s have explored XB-analogous concepts for unmanned systems, particularly in hypersonic testing, though without formal XB assignments under the post-1962 system. DARPA's programs, such as the Hypersonic Air-breathing Weapon Concept (HAWC) and Next Generation Responsive Strike (NextRS), have advanced drone prototypes for Mach 5+ strikes, often framed as experimental "Y-plane" demonstrators akin to historical XB bombers but designated as X-planes or internal project codes to emphasize multi-role versatility. Overall, fewer than five XB or equivalent designations have appeared since 1980, predominantly in non-military private ventures like the XB-1, reflecting a shift from government-led bomber experimentation to commercially funded, regulation-constrained innovation. Key challenges include Federal Aviation Administration (FAA) restrictions on overland sonic booms, established by the 1973 ban, which Boom addressed through NASA-partnered "boomless" research; funding has relied on private investment, with Boom raising over $270 million by 2023 from investors including United Airlines.

Legacy and Influence

Technological Contributions

The XB experimental bomber programs advanced aerodynamics, particularly for high-speed flight regimes. The Convair XB-58 demonstrated the efficacy of delta wings in achieving Mach 2 stability, allowing sustained supersonic performance without excessive structural stress or control issues, which directly informed the operational B-58 Hustler's design as the first U.S. supersonic bomber.^[42] Similarly, the North American XB-70 Valkyrie incorporated a fixed delta wing with hinged, drooping wingtips that folded downward during supersonic flight, capturing vortices to enhance directional stability and reduce induced drag by optimizing lift distribution.^[43] Its integrated compression inlets, embedded in the fuselage and wings, efficiently slowed incoming air for the engines while minimizing external drag at Mach 3, contributing to overall aerodynamic efficiency in extreme conditions.^[4] In propulsion, early XB aircraft pioneered jet engine integration and performance enhancements. The Douglas XB-43, as the first U.S. jet-powered bomber prototype, tested axial-flow turbojets like the General Electric TG-180 (later J35), validating compressor designs and thrust levels up to 4,000 lbf per engine under real-flight conditions and paving the way for multi-engine jet bombers. The Martin XB-51 further refined afterburner technology with its three General Electric J47-GE-13 turbojets, each capable of 5,200 lbf dry thrust and up to 7,000 lbf with afterburning, providing data on augmented thrust management that influenced subsequent tactical bombers and served as a precursor to more advanced thrust augmentation systems. Materials and systems innovations from XB programs addressed the challenges of high-speed, high-temperature environments. The XB-70 airframe relied extensively on titanium alloys and brazed stainless steel honeycomb panels to endure skin temperatures exceeding 600°F during Mach 3 cruise, enabling structural integrity without excessive weight penalties and informing heat-resistant designs for later supersonic aircraft.^[44] Evaluations of 1950s XB prototypes, including stability and control tests, laid foundational concepts for electronic flight control systems, highlighting the need for automated augmentation to handle unstable configurations at transonic and supersonic speeds, which evolved into early analog fly-by-wire implementations in the following decade.^[45] These contributions extended beyond prototypes, with XB testing accelerating transitions to production. For instance, endurance and range data from the Boeing XB-15 directly shaped the pressurized cabin and long-range fuel systems of the B-29 Superfortress.^[46] Scale and structural load insights from the Douglas XB-19 guided the oversized B-36 Peacemaker's development, validating massive wingspans and multi-engine configurations for intercontinental missions.^[47] Overall, XB programs reduced risks in scaling technologies, exemplified by the North American XB-45's rapid progression to the B-45 Tornado—first flight in 1947 followed by operational service in 1948—demonstrating efficient validation that shortened full development cycles for jet bombers.^[48]

Transition to Modern Designations

The transition to modern U.S. aircraft designations began with the adoption of the Tri-Service system on September 18, 1962, which unified naming conventions across the Department of Defense (DoD), Navy, and Air Force under the Military Specification for Aircraft Reporting (MSAR) Section II.^[7] This system replaced the service-specific prefixes like XB—used for experimental bombers since 1924—with a simplified Mission-Design-Series (MDS) format, where experimental aircraft received an "X-" prefix followed by a sequential number (e.g., the X-15 rocketplane), and bombers were designated in the B-series without experimental modifiers (e.g., B-1 Lancer and B-2 Spirit).^[7] The change aimed to eliminate redundancies and confusion from parallel service systems, standardizing all fixed-wing, rotary-wing, and unmanned vehicles under a single framework.^[7] The phasing out of the XB designation occurred gradually in the early 1960s, with the North American XB-70 Valkyrie marking the last official use in the military series during its flight tests from 1964 to 1969.^[4] After 1962, the DoD ceased assigning new XB numbers for official prototypes, though the term persisted informally in industry contexts, such as Boeing's internal labeling of conceptual designs to evoke historical experimental lineage.^[7] Post-1962 adaptations further diverged from the original XB convention; for instance, the Defense Advanced Research Projects Agency (DARPA) employs "XR-" prefixes for reconnaissance-focused experimental unmanned systems, as seen in the XRQ-73 SHEPARD hybrid-electric demonstrator unveiled in 2024.^[49] In the private sector, companies like Boom Supersonic revived the "XB" format for branding purposes, naming their 2024 supersonic demonstrator the XB-1 as a nod to early 20th-century experimental traditions.^[39] As of 2025, no active military XB designations exist, with the U.S. Air Force's Next Generation Air Dominance (NGAD) program reflecting this shift through unnumbered experimental phases leading to production models like the B-21 Raider bomber.^[7] The XB system's legacy encompasses over 50 designations assigned between 1924 and 1969, influencing modular design approaches in contemporary prototyping.^[7] Key policy evolution came via DoD Directive 4120.15, issued on November 24, 1971, which formalized sequential numbering and emphasized modularity to accommodate evolving mission requirements over rigid prototype sequencing.^[50]

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Jan 2, 2024 · The 500,000-pound plane's nose gear was blown back into the half-retracted gear well door, and the tires were also slashed. When they tried to ...
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Boom Supersonic XB-1 breaks sound barrier over Mojave Desert
Jan 29, 2025 · Boom Supersonic's XB-1 is a stepping stone in its plan to develop a commercially viable supersonic airliner, the Overture, capable of carrying ...
[40]
Convair B-58A Hustler - Air Force Museum
Convair built 116 B-58s: 30 test and pre-production aircraft and 86 for operational service. Hustlers flew in the Strategic Air Command between 1960 and 1970.
[41]
[PDF] Perspectives on Highly Adaptive or Morphing Aircraft
3. The XB-70's wing tips drooped, providing many benefits. At supersonic conditions the drooped tips improved directional stability and drag through capturing ...
[42]
XB-70 Valkyrie - NASA
Feb 28, 2014 · The XB-70 Valkyrie, with a planned cruise speed of Mach 3 and operating altitude of 70,000 feet, was to be the ultimate high-altitude, ...Missing: wire | Show results with:wire
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[PDF] fly-by-Wire systems enable safer, More efficient flight
“Digital fly-by-wire has unshackled designers from the rules of the 1950s and 1960s, so you end up with vehicles like the Space Shuttle, the B-2 bomber, and ...<|control11|><|separator|>
[44]
[PDF] NASA's First A - Aeronautics from 1958 to 2008
of building unique prototypes, such as the Douglas XB-19 (a very large prewar ... influence on, 19–20, 19n37 direction of research and project management ...
[45]
[PDF] 19730005255.pdf - NASA Technical Reports Server (NTRS)
In 1941 the Douglas XB-19, the world's largest land plane, and the Martin PB2M Mars, the world's largest flying boat, both in the 70 ton class, were flying.<|separator|>
[46]
[PDF] One Hundred Years of Flight - Air Force Historical Research Agency
A precursor of World War II bombers, the B–10 could fly faster than con- temporary pursuit aircraft and much faster than previous biplane and triplane bombers.
[47]
DARPA's X-plane Shepard UAS gets official designation, XRQ-73
Jun 25, 2024 · DARPA's Series Hybrid Electric Propulsion AiRcraft Demonstration (Shepard) programme aircraft received its official designation, XRQ-73.Missing: XR | Show results with:XR
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Designating and Naming Military Aerospace Vehicles - DTIC
This Directive reissues reference a DoD Directive 4120.15, Designating and Naming Military Aircraft, Rockets, and Guided Missiles, November 24, 1971 hereby ...