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 two XB-51 (46-686) made its first flight on 17 April 1950. It was accepted by the Air Force on 8 December 1951. Most of its initial 64 hours and 13 minutes of flight testing for Martin was flown by F.E. "Chris" Christopherson. Further Air Force flights were conducted by such noteworthy pilots as Guy Townsend, Chuck Yeager, and Russ Schleeh. By the end of its career, 686 had logged about 151 flight hours. The aircraft was used for gun, rocket, and bomb tests. It also performed high-speed flight trials.