Martin Model 234

XA-45, XB-51 Tactical Bomber
Variants/Specifications

After disappointing results with the "conventional" design of the XB-48, Martin produced a strikingly innovative airplane in the XB-51. Its sharply swept 53-foot wings supporting an improbably large 85-foot fuselage, the XB-51 continued a trend of Martin aerodynamics that went back to the short-winged B-26. Freed from the requirement to locate the XB-51's engines in wing nacelles as in the XB-48, Martin engineers mounted two General Electric J47 jets on pylons beneath the cockpit and a third beneath the plane's tail. Everything else went inside the fuselage, including fuel tanks that could be filled quickly from a single point, and a bicycle landing gear as in the XB-48. The XB-26H Stump Jumper was again pressed into service to test it. The XB-51 fuselage also contained a new bomb-bay invented by Martin engineers. Conventional bomb-bay doors, when opened at jet speeds, had caused buffeting as the airstream spilled into the internal cavity. In the new "rotary bomb-bay," bombs were mounted on the reverse side of the door. This was simply rotated from inside to outside when it came time to drop the bombs. Not only was the the new arrangement more steady at high speed, but extra bomb-bay units could be pre-loaded with weapons and quickly changed between missions.

Freed of nacelles, fuel tanks, landing gear wells, and external weapons points, the XB-51's wings were not only small but elegantly thin. In addition to advanced spoiler ailerons and slotted flaps, they incorporated a pioneer variable-incidence control system which allowed the whole wing to be rotated to different angles, from a 3 degree up-angle to 7 degrees - a handy feature in rotating for take off. The horizontal stabilizer, which was swept back at the same 35-degree angle as the wings, also had variable-incidence controls. To keep it free of the tail jet's exhaust, it was was mounted atop the rudder, an early application of the now familiar "T tail."

The XB-51 project originated in an Army Air Forces design competition in February 1946 for a new ground-support aircraft in succession to the Douglas A-26. Martin had won with a design for a large, heavily armed aircraft with a crew of six. Like the Mercator, the Martin XA-45 was to have a composite powerplant of two turboprop and two jet engines. Within weeks of the award, however, the AAF dropped the whole "Attack" category. New ground-support aircraft would be classified as Bombers; speed requirements would dictate all-jet power. Martin accepted the new requirements, and the project was redesignated XB-51. Two prototypes were ordered of a new design. The XB-51 would have a crew of only two: a pilot and an operator for the Shoran short-range navigation and bombing system. It would also carry up to 10,495 pounds of bombs and eight forward-firing 20 millimeter cannon with 1,280 rounds of ammunition.

Two prototype XB-51's were first flown and tested in late 1949 and early 1950, just before the outbreak of war in Korea. The Army soon called for a more modern attack plane to replace the aging B-26 (formerly A-26) Invader - especially one capable of attacking enemy supply lines at night. The new XB-51 seemed the obvious choice. Careful consideration, however, showed that it was really better suited to day missions over European battlefields, where the 200-mile range of the Shoran system would be no liability, and the XB-51's speed would allow it to evade enemy fighters. In Korea, where enemy MiG's had not yet appeared, a night intruder would not require speed so much as high maneuverability and the longest possible "loiter" time over the target.

Under increasing pressure from the Army, an Air Force board convened in October 1950 to evaluate a variety of possible night-intruder aircraft. Besides the XB-51, these included three North American designs - piston and turboprop versions of the Navy's AJ-1 Savage and a modernized B-45 jet bomber. The board also considered two planes from America's new NATO allies - the Canadian Avro CF-100 interceptor and English Electric Canberra. Although the Canberra was slower than the XB-51, its light wing loading (less than half the XB-51's 116.5 pounds per square foot) made it highly maneuverable at low speeds. It could loiter for two and a half hours over a target 900 miles from base, as opposed to only one hour 400 miles from base for the XB-51. After a fly-off of competing planes at Andrews Air Force Base in February 1951, the board recommended that an order be placed for 300 Canberras.

The XB-51 project was cancelled the following year. The two prototypes continued to fly, justifying some of their $12.5 million cost by participating in tests of their advanced aerodynamic and weapons-handling features. Both were eventually lost in crashes laid to pilot error.