The North American X-15 rocket-powered aircraft was part of the X-series of experimental aircraft, initiated with the Bell X-1, that were made for the USAF, NASA, and the USN . 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. It currently holds the world record for the fastest speed ever reached by a manned aircraft.
During the X-15 program, 13 of the flights (by eight pilots) met the USAF spaceflight criteria by exceeding the altitude of 50 miles (80.47 km. 264,000ft.), thus qualifying the pilots (Mike Adams, Joe Engle, Bob Rushworth, Pete Knight & Bob White) for astronaut status; some pilots also qualified for NASA astronaut wings (Bill Dana, Jack McKay, & Joe Walker).
Of all the X-15 missions, two flights (by Joe Walker) qualified as space flights, per the international FAI definition of a spaceflight by exceeding a 100 kilometer (62.137 mi, 328,084 ft) altitude.
Three X-15s were built, flying 199 test flights, the last on 24 October 1968. Twelve test pilots flew the X-15, among them were Neil Armstrong (first man on the moon) and Joe Engle (a space shuttle commander). In July and August 1963, pilot Joe Walker crossed the 100 km altitude mark twice, thus joining the NASA astronauts and Soviet Cosmonauts as the only men to have crossed the barrier into outer space (Alan Shepard was the first American in space, while Soviet Yuri Gagarin was the first human being in space).
U.S. Air Force Test pilot Maj. Michael J. Adams was killed, on 15 November 1967, in X-15 Flight 191 when his craft (X-15-3) entered a hypersonic spin while descending, then oscillated violently as aerodynamic forces increased after re-entry. As his craft's flight control system operated the control surfaces to their limits, the craft's acceleration built to ±15 degrees vertical and ±8 degrees lateral. The airframe broke apart at 60,000 ft altitude, scattering the craft's wreckage for 50 square miles. On 8 June 2004, a monument was erected at the cockpit's locale, near Randsburg, California. Maj. Adams was posthumously awarded astronaut wings for his final flight in craft X-15-3, which had reached 266,000 ft (81.1 km, 50.4 mi.) of altitude. In 1991, his name was added to the Astronaut Memorial monument, Kennedy Space Center, Florida.
The second X-15A was rebuilt after a landing accident. It was lengthened 2.4 ft (0.74 m), a pair of auxiliary fuel tanks attached under the fuselage, and a heat-resistant surface treatment applied. Re-named the X-15A-2, it first flew on 28 June 1964, reaching 4,520 mph (Mach 6.70) on October 3rd, 1967.