Martin XB-51

XB-51
First prototype, 46-685 during testing
Role	Bomber
Manufacturer	Glenn L. Martin Company
First flight	28 October 1949
Retired	25 March 1956
Status	Cancelled in 1952
Primary user	United States Air Force
Number built	2
Unit cost	US$12.6 million for the program

The Martin XB-51 was an American trijet ground attack aircraft. It was designed in 1945 and made its maiden flight in 1949. It was originally designed as a bomber for the United States Army Air Forces under specification V-8237-1 and was 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 XB-51 lost out in evaluation to the English Electric Canberra which entered service as the B-57.

The resulting unorthodox design, first flying on 28 October 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 in pods. The innovative, variable incidence wings, swept at 35° and with 6° anhedral, were equipped with leading-edge slats and full-width flaps. Spoilers gave most of the roll control and under-sized ailerons provided feel for the pilot. The combination of variable incidence adjustment and slotted flaps allowed for a shorter takeoff run. Four 954 lb (4.24 kN) thrust Rocket-Assisted Take Off (RATO) bottles with a 14-second burn duration could be fitted to the rear fuselage to improve takeoff performance. Spectacular launches were a feature of later test flights.

The main landing gear consisted of dual sets of wheels in tandem in the fuselage, similar to the B-47 Stratojet, with outrigger wheels at the wingtips (originally proved on a modified B-26 Marauder named "Middle River Stump Jumper"). The B-51 was a large but aerodynamically "clean" design which incorporated nearly all major systems internally. 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 (1,814 kg) bombload. Eight 20 mm (.79 in) cannons mounted in the nose would have been installed in production aircraft.