X-Planes

The X-planes are a series of experimental United States aircraft (and some rockets) used for testing of new technologies and usually kept highly secret during development.

The first of these, the Bell X-1, became well-known as the first plane to break the sound barrier, which it did in 1947. Later X-planes yielded important research results, but only the North American X-15 rocket plane of the early 1960s achieved comparable fame. X-planes 7 through 12 were actually missiles, and some other vehicles were unpiloted. Most X-planes are not expected to ever go into full-scale production, and usually only a few are produced. One exception is the Lockheed Martin X-35, which competed against the Boeing X-32 to become the Joint Strike Fighter.

As of 2007, new X-plane projects are still underway. The designation X-52 was skipped to avoid potential confusion with the operational B-52 Stratofortress strategic bomber.

** Information provided by Wikipedia **