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. As of September 2015, the X-15 holds the official world record for the highest speed ever recorded by a manned, powered aircraft. It could reach a top speed of 4,520 miles per hour (7,274 km/h), or Mach 6.72.
The X-15 was to explore the problems of ballistic flight, winged reentry, and gliding recovery from space. It was a stepping stone to later developments - either an X-15 launched atop Navaho G-26 boosters, an X-15 scramjet version, or the X-20 - that would lead to manned orbital spaceflight. This stepping-stone approach was abandoned and the crash programs of Mercury and Apollo initiated instead, using ballistic capsules for crew recovery. Once these projects were over America returned to its original course and developed the winged space shuttle as its manned spacecraft.