Aircraft, awful and awesome: Martin XB-51

Dr. Marshall Michel
86th Airlift Wing historian

***image1***In February 1946 the Army Air Forces asked for designs for a
low-altitude attack aircraft, a type that had been very successful in
the Pacific theater in World War II.

The Glenn L. Martin Company proposed the XB-51, an aircraft with thin, small, droopy wings, three General Electric J47 jet engines – two on pylons beneath the cockpit and a third beneath the plane’s tail – and an enormous 85-foot fuselage carrying all the aircraft’s systems, including fuel and a “bicycle” landing consisting of tandem dual main wheels which retracted into the fuselage.

It also had an inventive “rotary” bomb-bay that had the bombs mounted directly on the door and simply flipped over just before bomb release, avoiding the drag and buffeting of bomb doors and an open bomb bay.

This made the XB-51 a very steady high-speed delivery platform and allowed extra bomb-bay units to be pre-loaded with weapons and quickly changed between missions.

The XB-51’s sharply swept small wings, with a six-degree downward droop, were especially innovative. They did not use ailerons but rather “spoilers” on the upper surface of the wings for direction control.

The wings also slid up and down from a three-degree up-angle to a seven-degree down-angle to help with take off and landings, a feature called variable-incidence. Its horizontal stabilizer was a revolutionary (for the time) “T tail.”

The XB-51’s crew consisted of a pilot and a navigator seated behind him in the fuselage, operating the SHORAN navigation and bombing system developed for all weather operations in Europe.

The aircraft carried a very heavy forward firing battery of eight 20-millimeter cannon for strafing and 4,000 pounds of  bombs.

Two prototype XB-51’s were tested in late 1949 and early 1950, and the first flights belied the old adage, “if it looks good, it flies good.” Air Force test pilots found the XB-51 very fast, easy to handle at low altitude, and noted the design would require relatively few modifications.

Its high speed made it immune to enemy fighters and its relatively short range was not an issue for operations in Europe.

But the ongoing Korean War put the Air Force under pressure from the Army to develop an aircraft capable of attacking enemy supply lines at night, a mission that did not require speed but rather high maneuverability at slow speed and long range.

The Air Force looked at the British Canberra twin engine jet bomber, which was 100 knots slower than the XB-51 but much more maneuverable and had great endurance. In a fly-off in early 1951, the Canberra was declared the winner and the XB-51 project, despite its superb performance and its obvious utility in Europe, was cancelled.