| Martin XB-51
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| Description
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| Role | ground attack
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| Crew | two; pilot and navigator in tandem
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| First flight | April 17, 1950
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| Manufacturer | Martin
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| Dimensions
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| Length | 85 ft 1 in | 25.9 m
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| Wingspan | 53 ft 1 in | 16.2 m
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| Height | 17 ft 4 in | 5.3 m
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| Wing area | 548 ft² | 50.9 m²
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| Weights
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| Empty | 29,584 lb | 13,419 kg
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| Loaded | 55,923 lb | 25,366 kg
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| Maximum takeoff | 62,457 lb | 28,330 kg
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| Powerplant
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| Engines | 3 × General Electric J47-GE-13 turbojets
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| Thrust | 5200 lb | 23 kN
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| Performance
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| Maximum speed | 645 mph | 1038 km/h
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| Combat range | 1075 miles | 1,730 km
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| Ferry range | 1613 miles | 2,596 km
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| Service ceiling | 40,500 ft | 12,300 m
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| Rate of climb | 6980 ft/min | 2,130 m/min
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| Avionics
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| Avionics |
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| Armament
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| Guns | 8 × 20 mm cannon with 1280 rounds
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| Bombs | 4 × 1600 lb (730 kg) bomb internally, or 2 × 2000 lb (900 kg) bombs externally. Maximum bombload 10,400 lb (4,700 kg).
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| Rockets | 8 × High Velocity Aerial Rockets (HVAR)
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The Martin XB-51 was a ground attack aircraft designed to a 1945 United States Army Air Force requirement, and originally designated XA-45. The "A" ground attack classification was eliminated the next year, and the XB-51 designation was assigned instead. The requirement was for low-level bombing and close support.
The resulting design, first flying on October 28, 1949 was (unusually for a combat aircraft) fitted with three engines, General Electric J47s in this case; one at the extreme tail with an intake at the base of the tailfin, and two underneath the forward fuselage. The wings, swept at 35° and with 6° anhedral, were equipped with variable incidence, leading-edge slots, full-width flaps and spoilers instead of ailerons. The main landing gear was dual sets of wheels in tandem in the fuselage, similar to the B-47 Stratojet, with outrigger wheels at the wingtips. The aircraft was fitted with a rotating bomb bay, a Martin trademark; bombs could also be carried externally up to a maximum load of 10,400 lb (4,700 kg), although the specified basic mission only required a 4,000 lb (1,800 kg) bombload.
In 1950, the United States Air Force issued a new requirement based on early Korean war experience for a night intruder/bomber to replace the A-26 Invader. The XB-51 was entered, as well as the Avro Canada CF-100 and the English Electric Canberra. The Canberra and XB-51 emerged as the favorites. The XB-51 was a highly maneuverable aircraft at low level, and substantially faster than the Canberra. However, its load limiting factor of only 3.67 G (36 m/s²) restricted tight turns, and the XB-51's endurance was substantially poorer than the Canberra's; this latter proved to be the deciding factor. Additionally, the tandem main gear plus outriggers of the XB-51 was thought unsuitable for the requirement to fly from emergency forward airfields. The Canberra was selected for procurement and the XB-51 program ended. Martin did not end up the loser, however, for they were selected to build the 250 Canberras ordered under the designation B-57A.
Flight testing for research purposes continued after program cancellation. The second prototype crashed on May 9, 1952 during low-level aerobatics. The other aircraft continued to fly, including appearing in the film Towards the Unknown, until it crashed on takeoff on March 25, 1956.
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