The flights of the North American X-15, an experimental American spaceplane built by North American Aviation and operated by the United States Air Force and NASA, were conducted from 1959-1968. Twelve pilots flew three X-15 spaceplanes, flying record high-altitude flights, high-speed flights, and sub-orbital spaceflights. Collectively, pilots and craft performed a total of 199 free flights after being carried aloft and then air launched from one of two modified B-52 mother ships. The pilots and craft also performed twelve scheduled captive carry test flights and 125 aborted flights (frequently due either to technical problems or poor weather) in which the X-15 did not uncouple from its B-52 mother ship, for a grand total of 336 flights. The X-15 program's flights generated data and flight experience which supported future development of aircraft, spacecraft, and human spaceflight. Five principal aircraft were used during the X-15 program: three X-15 spaceplanes and two modified "nonstandard" NB-52 bomber carriers: 56-6670, 56-6671, 56-6672, 52-003 nicknamed The High and Mighty One, 52-008 nicknamed The Challenger, later Balls 8. Additionally, F-100, F-104 and F5D chase aircraft and C-130 and C-47 transports supported the program. Pilots Twelve pilots flew the X-15 over the course of its career. Scott Crossfield and William Dana flew the X-15 on its first and last free flights, respectively. Joseph Walker set the program's top two altitude records on its 90th and 91st free flights (347,800 and 354,200 feet, respectively), becoming the only pilot to fly past the Kármán line, the 100 kilometer, FAI-recognized boundary of outer space, during the program. William Knight set the program's Mach (6.70) and speed (4,520 mph) records on its 188th free flight. Neil Armstrong was the first pilot to fly the program's third plane, the X-15-3. Following his participation in the program, Joe Engle commanded a future spaceplane, the Space Shuttle, on two missions. Robert Rushworth flew 34 free flights, the most in the program. Forrest Petersen flew five, the fewest. Robert White was the first person to fly the X-15 above 100,000 feet. Milton Thompson piloted a series of typical flights during the middle of the program. John McKay was injured in (and recovered from, returning to active flight status) a landing accident which damaged the X-15-2, leading to its refurbishment as the modified X-15A-2. Michael Adams was killed in the program's 191st free flight. Five pilots were Air Force personnel, five were NASA personnel, one (Crossfield) was