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 crewed, powered aircraft, set in October 1967 when William J. Knight flew at Mach 6.70 at 102,100 feet (31,120 m), a speed of 4,520 miles per hour (7,274 km/h; 2,021 m/s), and has remained unbroken as of 2020.