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, the NASA, and the USN. 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. 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 for astronaut status; some pilots also qualified for NASA astronaut wings.   Of all the X-15 missions, two flights (by the same pilot) qualified as space flights, per the international FAI definition of a spaceflight by exceeding a 100 kilometer (62.137 mi, 328,084 ft) altitude.
The X-15 is based on a concept study from Walter Dornberger for the NACA for a hypersonic research aircraft.  The requests for proposal were published on 30 December 1954 for the airframe and on 4 February 1955 for the rocket engine. The X-15 was built by two manufacturers: North American Aviation was contracted for the airframe in November 1955, and Reaction Motors was contracted for building the engines in 1956.
The first X-15 flight was an unpowered test flight by Scott Crossfield, on 8 June 1959; he also piloted the first powered flight, on 17 September 1959, with his first XLR-99 flight on 15 November 1960.
Like most X-series aircraft, the X-15 was designed to be carried aloft, under the wing of a B-52 bomber plane. The X-15 fuselage was long and cylindrical, with rear fairings that flattened its appearance, and thick, dorsal and ventral wedge-fin stabilizers. Parts of the fuselage were heat-resistant nickel alloy (Inconel-X 750). The retractable landing gear comprised a nose-wheel carriage and two rear skis. The skis did not extend beyond the ventral fin, which required the pilot to jettison the lower fin (fitted with a parachute) just before landing. The two XLR-11 rocket engines for the initial X-15A model delivered 72kN (16,000 lbft) of total thrust; the main engine (installed later) was a single XLR-99 rocket engine delivering 254kN (57,000 lbft) at sea level, and 311 kN (70,000 lbft) at peak altitude.
Before 1958, USAF and NACA, (later NASA), officials discussed an orbital X-15 spacecraft — the X-15B — for launching to outer space atop an SM-64 Navajo missile, that was cancelled when the NACA became the NASA, and Project Mercury was approved. By 1959, the X-20 Dyna-Soar space-glider program became the USAF's preferred means for launching military manned-spacecraft into orbit; the program was cancelled in the early 1960s.
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 7,274 km/h (4,520 mph, 2,021 m/s).
The altitudes attained by the X-15 aircraft do not match that of Alan Shephard's 1961 NASA spacecapsule flight (116 miles), nor subsequent NASA spacecapsules and space shuttle flights. However, the X-15 flights did reign supreme among rocket-powered aircraft until the third spaceflight of Space Ship One in 2004. The widely-reported record achieved, by the small X-43A scramjet testbed, on 16 November 2004, of approximately Mach 10 (6,600 mph, 10,622 km/h, 2.95 km/s) at 95,000 ft (29 km, 17.99 mi) is an air-breathing jet engine record.
Five aircraft were the X-15 program: three X-15s, two B-52 bombers:
A 200th flight over Nevada was slated for 21 November 1968, piloted by William J. Knight. Technical problems and bad weather delayed the flight six times, and on 20 December 1968, the 200th flight was finally cancelled. The X-15 was unfastened from the wing of bomber NB-52A, and prepared for indefinite storage.
In the United States there are two definitions of how high a person must go to be referred to as an astronaut. The USAF decided to award astronaut wings to anyone who achieved an altitude of 50 miles (80.47 km) or more. However the FAI set the limit of space at 100 km. Thirteen X-15 flights went higher than 50 miles (80.47 km) and two of these reached over 62.137 miles (100 km).
|Flight 62||17 July 1962||3,831 mph||59.6 miles||Robert M. White|
|Flight 77||17 January 1963||3,677 mph||51.4 miles||Joe Walker|
|Flight 87||27 June 1963||3,425 mph||53.9 miles||Robert Rushworth|
|Flight 90||19 July 1963||3,710 mph||65.8 miles||Joe Walker|
|Flight 91||22 August 1963||3,794 mph||67.0 miles||Joe Walker|
|Flight 138||29 June 1965||3,431 mph||53.1 miles||Joseph H. Engle|
|Flight 143||10 August 1965||3,549 mph||51.3 miles||Joseph H. Engle|
|Flight 150||28 September 1965||3,731 mph||55.9 miles||John B. McKay|
|Flight 153||14 October 1965||3,554 mph||50.4 miles||Joseph H. Engle|
|Flight 174||1 November 1966||3,750 mph||58.1 miles||Bill Dana|
|Flight 190||17 October 1967||3,856 mph||53.1 miles||Pete Knight|
|Flight 191||15 November 1967||3,569 mph||50.3 miles||Michael J. Adams†|
|Flight 197||21 August 1968||3,443 mph||50.6 miles||Bill Dana|
|Flight 45||9 November 1961||4,092 mph||19.2 miles||Robert M. White|
|Flight 59||27 June 1962||4,104 mph||23.4 miles||Joe Walker|
|Flight 64||26 July 1962||3,989 mph||18.7 miles||Neil Armstrong|
|Flight 86||25 June 1963||3,910 mph||21.7 miles||Joe Walker|
|Flight 89||18 July 1963||3,925 mph||19.8 miles||Robert Rushworth|
|Flight 97||5 December 1963||4,017 mph||19.1 miles||Robert Rushworth|
|Flight 105||29 April 1964||3,905 mph||19.2 miles||Robert Rushworth|
|Flight 137||22 June 1965||3,938 mph||29.5 miles||John B. McKay|
|Flight 175||18 November 1966||4,250 mph||18.7 miles||Pete Knight|
|Flight 188||3 October 1967||4,519 mph||36.3 miles||Pete Knight|
|X-15 pilots and their achievements during the program|
|Michael J. Adams†||U.S. Air Force||7||1||0||5.59||3,822||50.3|
|Scott Crossfield||North American Aviation||14||0||0||2.97||1,959||15.3|
|Joseph H. Engle||U.S. Air Force||16||3||0||5.71||3,887||53.1|
|Pete Knight||U.S. Air Force||16||1||0||6.70||4,519||53.1|
|John B. McKay||NASA||29||0||0||5.65||3,863||55.9|
|Forrest S. Petersen||U.S. Navy||5||0||0||5.3||3,600||19.2|
|Robert A. Rushworth||U.S. Air Force||34||1||0||6.06||4,017||53.9|
|Joe Walker||U.S. Air Force||25||3||2||5.92||4,104||67.0|
|Robert M. White||U.S. Air Force||16||1||0||6.04||4,092||59.6|
|† Killed * White was backup for Capt. Iven Kincheloe|