The North American X-15 is an American hypersonic aircraft. It was 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 X15 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. The X-15's highest speed, 4,520 miles per hour (7,274 km/h; 2,021 m/s), was achieved on 3 October 1967, when William J. Knight flew at Mach 6.7 at an altitude of 102,100 feet (31,120 m), or 19.34 miles. This set the official world record for the highest speed ever recorded by a crewed, powered aircraft, which remains unbroken.
The North American X-15 rocket-powered research aircraft bridged the gap between manned flight within the atmosphere and manned flight beyond the atmosphere into space. After completing its initial test flights in 1959, the X-15 became the first winged aircraft to attain velocities of Mach 4, 5, and 6 (four, five, and six times the speed of sound). Because of its high-speed capability, the X-15 had to be designed to withstand aerodynamic temperatures on the order of 1,200 degrees F.; as a result, the aircraft was fabricated using a special high-strength nickel alloy named Inconel X.

Air-launched from a modified Boeing B-52 Stratofortress aircraft, the X-15 required conventional aerodynamic control surfaces to operate within the atmosphere and special "thruster" reaction control rockets located in the nose and wings of the aircraft to enable the pilot to maintain control when flying on the fringes of space. Indeed, the X-15 design was so much like that of a space vehicle that during the formative days of Project Mercury, America’s first attempt to put a man in orbit, North American and National Air and Space Administration (NASA) engineers gave serious consideration to utilizing a growth version of the X-15 for the manned orbiting mission. This plan was dropped in favor of using a blunt-body reentry vehicle. Because of the potential dangers to the pilot should the X-15’s pressurized cockpit lose its atmosphere while the aircraft operated in a near-space environment, X-15 pilots wore specially developed full-pressure protection ‘spacesuits’ while flying the experimental plane.

Three X-15 research aircraft were built and flown, completing a total of 199 research flights. The National Air and Space Museum has the historic X-15 #1, Air Force serial 56-6670. The X15 #2 (56-6671) was rebuilt following a landing accident as the advanced X-15A-2, having increased propellant capacity and, hence, a higher potential performance. The X-15A-2 was the fastest X-15 flown, and it is now on exhibit at the Air Force Museum, Wright-Patterson Air Force Base, Ohio. The X-15 #3 (56-6672) featured an advanced cockpit display panel and a special adaptive control system. The aircraft made many noteworthy flights until it crashed during atmospheric reentry, following pilot disorientation and a control-system failure. The pilot, Capt. Michael Adams, was killed.

General characteristics:

Crew: One
Length: 50 ft 9 in (15.47 m)
Wingspan: 22 ft 4 in (6.81 m)
Height: 13 ft 3 in (4.04 m)
Wing area: 200 sq ft (19 m2)
Empty weight: 14,600 lb (6,622 kg)
Gross weight: 34,000 lb (15,422 kg)
Powerplant: 1 × Reaction Motors XLR99-RM-2 liquid-fuelled rocket engine, 70,400 lbf (313 kN) thrust
Performance

Maximum speed: 4,520 mph (7,270 km/h, 3,930 kn)
Range: 280 mi (450 km, 240 nmi)
Service ceiling: 354,330 ft (108,000 m)
Rate of climb: 60,000 ft/min (300 m/s)
Thrust/weight: 2.07

Archive footage courtesy of NASA (nasa.gov)

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