Three X-15 hypersonic research aircraft were built to furnish data with regard to heating stability, and control at high speeds and altitudes, as well as the problems of re-entry into the atmosphere. The rocket-powered vehicles were constructed mostly of titanium and stainless steel, with a nickel alloy (Inconel X) skin. Launched from a B-52 mothership, they were capable of exceeding 50 miles in altitude, which is considered the edge of space.

The number three X-15, (56-6672), made its first flight on 20 December 1961 with Neil A. Armstrong at the controls. Robert M. White used the aircraft to set the FAI world altitude record on 17 July 1962. He attained 314,750 feet at a speed of Mach 5.45, earning his astronaut wings in the process. Other pilots who earned their astronaut wings in '672 include Joseph A. Walker, Robert A. Rushworth, Joe H. Engle, John B. McKay, William H. Dana, William "Pete" Knight, and Michael J. Adams. Walker set an unofficial altitude record on 22 August 1963 when he flew '672 to 354,200 feet. Ship three flew a total of 65 flights during the course of the program. It was lost in an accident on 15 November 1967 that claimed the life of Maj. Michael J. Adams.