X (experimental) airplanes were built by aviation companies but were flown and maintained by NASA (or its predecessor, NACA), and the military. These planes, most of which were one-of-a-kind, were used to explore high-speed, high-altitude flight, as well as vertical take offs and launchings, new materials such as titanium, and variable-geometry wings. Here’s a look at the first 13 years of X airplanes, going from 1946 through 1959.
For more information and images of the X-15, click here.