1. Michael J. Adams, U.S. Air Force, 7 flights.
2. Neil Armstrong, NASA, 7 flights. He reached an altitude of 207,500 feet in the X-15-3 and a
speed of Mach 5.74 (3,989 mph) in the X-15-1.
3. A. Scott Crossfield, North American Aviation, 14 flights.
4. William H. Dana, NASA, 16 flights.
5. Joe H. Engle, U.S. Air Force, 16 flights.
6. William J. Knight, U.S. Air Force, 16 flights.
7. John B. McKay, NASA, 29 flights.
8. Forrest S. Petersen, U.S. Air Force, 5 flights.
9. Robert A. Rushworth, U.S. Air Force, 34 flights.
10. Milton O. Thompson, NASA, 14 flights.
11. Joseph A. Walker, NASA, 25 flights.
12. Robert M. White, U.S. Air Force, 16 flights.

The North American X-15 was a rocket powered aircraft operated by the United States Air Force and the Nataional AeronauticsSpace Administration (NASA) as part of the X-plane series of experimental aircraft. The X-15 set speed and altitude records in the early 1960s, reaching the edge of outer space and returning with valuable data used in aircraft and spacecraft design. As of 2012, the X-15 holds the official world record for the fastest speed ever reached by a manned aircraft.

During the X-15 program, 13 different flights by eight pilots met the USAF spaceflight criterion by exceeding the altitude of 50 miles (80 km) thus qualifying the pilots for astronaut status. The USAF pilots qualified for USAF astronaut wings, while the civilian pilots were awarded NASA astronaut wings in 2005, 35 years after the last X-15 flight.

Of all the X-15 missions, two flights (by the same pilot) qualified as space flights per the international (Federation AeronautiqueInternationale) definition of a spaceflight by exceeding 100 kilometers (62.1 mi, 328,084 ft) in altitude.