Rear view of a North American X-15 hanging in the Main Hall of the Smithsonian Air and Space Museum in Washington, D.C.

The X-15 was part of the "X" series of experimental aircraft built by the National Advisory Committee for Aeronautics -- the federal civilian agency founded in 1915 to conduct research into aviation and space flight. (In 1958, NACA would be folded into the newly-founded NASA.)

The concept was developed by Major General (ret.) Dr. Walter Dornberger, a German artillery officer who helped build the V-2 ballistic missile for Nazi Germany during World War II. (Dornberger spent two years in a British prison for using slave labor to build the V-2. He was later patriated to the United States, where he worked for the U.S. Air Force for three years building missiles. From 1950 to 1960 he worked for Bell Aircraft.) NACA published its request for proposals in December 1954 and February 1955. North American Aviation bult the airframe in November 1955, and Reaction Motors built the engines in 1956.

Three X-15s were built. The first flew an unpowered flight on June 8, 1959, and the last flew on October 24, 1968.

The X-15 was carried aloft under the wing of a B-52. It was released 8.5 miles up. Its rocket engine fired for just 80 seconds, but this was enough to reach speeds greater than Mach 3. It glided to the ground unpowered, and its landing gear were a nosewheel and two rear skis. (The rudder below the aircraft had to be jettisoned before landing.) The engine used ammonia and liquid oxygen for propellant, and could be throttled up and down (unlike other rockets, where were just "on/off"). There were the first throttled rocket rated for use in a manned aircraft.

Much of the X-15's powered and unpowered flight occurred in regions of the atmosphere were there was too little air to allow conventional control surfaces to work. So the X-15 used rocket thrusters to control its pitch, yaw, and roll. Later versions of the X-15 used a computer to given extra inputs to the thruster rockets that helped stabilize the aircraft. (This additional system was used only during powered and the highest flight, then automatically turned off.)

Some X-15s had just a single joystick, but others were fitted with three sticks (to provide additional fine control). The windows were heated to prevent icing, and a headrest in front of the pilot helped avoid injury during high decceleration. The X-15 had an ejection seat (never used) that allowed ejection at speeds up to Mach 4. Fins helped stabilize the ejection seat and guide it to the ground; they were jettisoned automatically and a parachute deployed for a soft-landing. Pilots wore a suit pressurized with nitrogen gas, the cockpit was pressurized with nitrogen gas (above 35,000 feet), and the pilot had a dedicated oxygen supply.

The X-15 set numerous speed and altitude records, and was the first manned "spaceplane" to reach outer space. As of 2012, the X-15 still holds the world record for the fastest speed ever reached by a manned rocket-powered aircraft.

NASA says that "outer space" begins 50 miles above the earth's surface. By that definition, 13 flights by eight pilots reached outer space.

The Fédération Aéronautique Internationale, however, defines outer space as beginning at 62.1 miles (100 km). By that definition, two flights by pilot Joseph A. Walker reached outer space.

This makes the X-15 the world's first operational spaceplane.