The North American X-15 was a hypersonic rocket-powered aircraft operated by the United States Air Force and the National Aeronautics and Space Administration as part of the X-plane series of experimental aircraft. The X-15 set speed and altitude records in the 1960s, reaching the edge of outer space and returning with valuable data used in aircraft and spacecraft design. The X-15's official world record for the highest speed ever recorded by a manned, powered aircraft, set in October 1967 when William J. Knight flew Mach 6.72 at 102,100 feet (31,120 m), a speed of 4,520 miles per hour (7,274 km/h; 2,021 m/s), has remained unchallenged as of December 2017.

During the X-15 program, 13 flights by eight pilots met the Air Force spaceflight criterion by exceeding the altitude of 50 miles (80 km), thus qualifying these pilots as being astronauts. The Air Force pilots qualified for astronaut wings immediately, while the civilian pilots were eventually awarded NASA astronaut wings in 2005, 35 years after the last X-15 flight. The only Navy pilot in the X-15 program never took the aircraft above the requisite 50 mile altitude and so as a result, never earned astronaut wings.

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