The Martin XB-51 was a three-engine, swept-wing, light bomber. It carried a two-man crew and had a top speed of 645mph at sea level. Only two prototypes were built: 46-685 and 46-686. The aircraft was never put into production, and the Glenn L. Martin Company was directed to build the B-57 instead.

The number one XB-51 (46-685) was rolled out from the Martin plant in Baltimore, Maryland on 4 September 1949. On 19 October, a tri-service inspection and taxi demonstration was conducted by Martin's chief test pilot O. E. "Pat" Tibbs. Tibbs flew the aircraft for the first time on 27 October 1949. The 34-minute flight ended with a landing at NAS Patuxent River. The following day, the aft main gear collapsed, damaging the gear doors.

Phase I (Contractor) evaluations included 45 flights. Phase II testing, under the auspices of the Air Force Air Materiel Command, began on 31 March 1950. On 27 September, the aircraft was damaged in a hard landing. After 41 Air Force flights at Martin's Baltimore facility, Phase II testing was completed on 10 November 1950.

Next, Martin conducted Phase III (Contractor Design Refinement) tests. After 39 flights, the aircraft was returned to the Air Force. In February 1951, a fly-off competition between the XB-51 and the English Electric Canberra at Andrews AFB, Maryland resulted in Air Force selection of the Canberra (to be built as the B-57). The XB-51 project was cancelled the following year, but the two prototypes continued to fly.

On landing at Wright-Patterson AFB, Ohio on 28 February 1952, the main gear collapsed, and the XB-51 had to be trucked back to Baltimore for repairs.

With repairs complete, the XB-51 was ferried to Edwards AFB, California for high-speed bombing tests.

In March 1956, the Martin XB-51, tail number 6685 was repainted as the "Gilbert XF-120," tail number 6684. In this paint scheme, it was used in the motion picture Toward The Unknown.