Mach 6.70

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 highest speed, 4,520 miles per hour (7,274 km/h; 2,021 m/s), was achieved in October 1967, when William J. Knight flew at Mach 6.70 at an altitude of 102,100 feet (31,120 m), or 19.34 miles. This set the official world record for the highest speed ever recorded by a crewed, powered aircraft, which remains unbroken.

Three X-15s were built, flying 199 test flights, the last on 24 October 1968. Like many X-series aircraft, the X-15 was designed to be carried aloft and drop launched from under the wing of a B-52 mother ship.
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Text-Source: Wikipedia