This stamp commemorates X-Planes, a series of experimental vehicles involving the U.S. military, NASA and companies such as Bell Aircraft, Boeing, Northrop and Lockheed Martin. From the first X-Plane flight in 1946 to the present, a variety of vehicles - manned and unmanned aircraft, cruise missiles, gliders and spacecraft - has helped extend the nation's reach into space.
Extraordinary technological advances following World War II prompted many different X-Plane concepts, including the X-1, a rocket-powered plane. On Oct. 14, 1947, with Air Force test pilot Chuck Yeager at the controls, it became the first plane to intentionally travel faster than Mach 1.0, the speed of sound. In 1956, the X-2 became the first plane to reach an altitude of more than 100,000 feet (nearly 19 miles) and set a new speed record of Mach 2.87.
One of the best known - and the fastest and highest-flying, winged vehicle - was the X-15. By the late 1960s, it had flown at speeds exceeding 4,500 miles per hour and reached an altitude of more than 60 miles. Eight X-15 pilots earned astronaut wings for achieving an altitude of 50 miles or more as a result of their flights in this plane.