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.

The X-15 flew faster and higher than any other airplane. A peak altitude of 354,200 feet (67± miles) was reached by the X-15, and the X-15A-2 attained a speed of Mach 6.72 (4,534 mph) while testing a new ablative thermal protection material and a proposed design for a hypersonic ramjet. Various proposals were set forth for modifying the aircraft to accomplish new and even more radical tasks. At one point, NASA scientists planned to test a hydrogen-fueled supersonic combustion ramjet engine mounted on the X-15s lower vertical fin. A mock-up of this proposed installation was flight-tested on the X-15A-2. Other ideas included modifying the X-15 with a slender delta wing and using the aircraft as a booster for small satellite launch vehicles. None of these ideas, however, came to fruition.

The X-15 spearheaded research in a variety of areas: hypersonic aerodynamics, winged reentry from space. life-support systems for spacecraft, aerodynamic heating and heat transfer research, and earth sciences experiments. A total of 700 technical documents were produced, equivalent to the output of a typical 4,000-man federal research center for more than two years.

Development of the X-15 began in 1954, in a joint research program sponsored by the National Advisory Committee for Aeronautics (forerunner of NASA), the U.S. Air Force. the U.S. Navy, and private industry. North American was selected as prime contractor on the project following a competition in which Douglas. Republic. and Bell also participated. By the time of its first airborne test, flight research was too complex to rely on simple air-to-ground communications near a test field. The Air Force and the National Advisory Committee for Aeronautics developed a special 485-mile-long test corridor stretching from Wendover Air Force Base. Utah. to Edwards Air Force Base. California. It was planned that the X15 would be air-launched from a Boeing B-52 near Wendover. then fly down this corridor, the High Range. to Edwards. monitored by tracking stations at Ely and Beatty. Nevada. and at Edwards. The range lay along a series of flat dry lakes. where the X-15 could make an emergency landing. if necessary. Nothing this extensive had previously existed in flight research, and it foreshadowed the worldwide tracking network developed by American manned spacecraft ventures. The X-15 would complete its research mission and then. followed by special Lockheed F-104 chase aircraft. would land on the hard clay of Rogers (formerly Muroc) Dry Lake. Because the X-15 featured a cruciform tail surface arrangement. it was necessary for the designers to make the lower half of the ventral fin jettisonable prior to landing so that the conventional two-wheel, nose-landing gear and two tail-mounted landing skids could support the aircraft.