Late in the year of 1954, the National Advisory Committee for Aeronautics (NACA) issued a requirement for an air-launched manned research vehicle with a maximum speed of more than Mach 6 and a maximum altitude of more than fifty miles. North American Aviation Incorporated was awarded the contract for the new research vehicle, the X-15.

Developed under USAF Project MX-1226, three X-15s were built and together made 199 flights during a research program which lasted from 1959 to 1968, exceeding all speed and altitude goals set. The X-15 was designed to fly at speeds of Mach 6 and altitudes up to 250,000 feet. The aircraft went on to reach a maximum speed of Mach 6.7 and a maximum altitude of 354,200 feet. Mach 6 is about one mile per second and flight above 265,000 feet qualifies an Air Force pilot as an astronaut.

The X-15 made its public appearance on 15 October 1958 at North American's Los Angeles facility and the first of the three X-15s arrived at Edwards AFB two days later. On 10 March 1959, with project pilot A. Scott Crossfield at the controls, the X-15 made its first captive-carry flight under the right wing of a B-52 carrier aircraft. On 8 June 1959, it made its first glide flight, reaching a speed of 522mph (Mach 0.79) from a release altitude of 52,341 feet.

During a test flight, the X-15 would be air-launched by NASA's converted B-52 at an altitude of 45,000 feet and a speed of 500 mph. Generally, there were two types of flight profiles: high-speed or high-altitude. High-speed flights were usually conducted below an altitude of 100,000 feet and flown as a conventional airplane using aerodynamic controls. High-altitude flights began with a steep, full-power climb to leave the atmosphere. This was followed by up to two minutes of "coasting up" to peak altitude after the engine was shut down. High-altitude flights usually lasted for 2-5 minutes as it made a ballistic arc before reentering the atmosphere. A reaction control system, employing hydrogen peroxide thrusters located on the nose and wings, was used to maintain attitude above the atmosphere.

A typical research flight lasted about 10 or 11 minutes while covering nearly 400 miles along a course that stretched from Smith Ranch, Nevada to Edwards Air Force Base.

The X-15 program made many accomplishments, here is list of some of its contributions to space flight:

• First use of a full-pressure suit for spaceflight.
• First use of reaction controls for maneuvering in space.
• First use of a flight control system that automatically blended aerodynamic and reaction controls.
• Development of thermal protection for hypersonic reentry.
• Development of the first large, restartable, and throttleable rocket engine.
• Development of an inertial guidance system.
• Demonstration of a pilot's ability to operate in "micro-gravity".
• Demonstration of the first piloted reentry-to-landing from space.
• Acquisition of hypersonic acoustic measurements, which influenced structural design criteria for Mercury capsule.
• Verification of the validity of hypersonic wind tunnel data, which were later used in the design of the Space Shuttle.