The North American X-15 remains the fastest manned aircraft ever built, reaching a record-breaking speed of Mach 6.70. Developed in the 1960s as a joint project between NASA and the U.S. Air Force, this rocket-powered plane was designed to explore the edge of space and the effects of hypersonic flight. ​Flying at 4,520 miles per hour (7,270 km/h), the X-15 reached altitudes exceeding 50 miles, which officially qualified several of its pilots as astronauts. This experimental aircraft provided cri