The X-planes concept officially came into being in 1944, as a joint programme between the National Advisory Committee for Aeronautics, the US Navy and the US Army Air Force, in order to pursue research into high-speed aircraft. NACA later became the National Aeronautics and Space Administration and the USAAF became the United States Air Force. Other organizations such as the Defence Advanced Research Projects Agency and the US Marine Corps have also since sponsored X-plane projects. The first experimental aircraft specification, for a transonic rocket plane, was placed in 1945, and the first operational flight of an X-plane took place when the Bell X-1 made its first powered flight nearly three years later at Muroc Air Force Base, California, now known as Edwards Air Force Base. The majority of X-plane testing has since taken place there. X-planes have since accomplished many aviation "firsts" including breaking speed and altitude barriers, varying wing sweepin flight, implementing exotic alloys and propulsion innovations, and many more. New X-planes appeared fairly regularly for many years until the flow temporarily stopped in the early 1970s. A series of experimental hypersonic projects, including an advanced version of the Martin Marietta X-24lifting body, were turned down. Eventually issues with the Rockwell HiMAT advanced UAV led to a manned X-plane with forward sweep, the Grumman X-29, which flew in 1984. Some of the X-planes have been well publicized, while others, such as the X-16, have been developed in secrecy. The first, the Bell X-1, became well known in 1947 after it became the first aircraft to break the sound barrier in level flight. Later X-planes supported important research in a multitude of aerodynamic and technical fields, but only the North American X-15rocket plane of the early 1960s achieved comparable fame to that of the X-1. X-planes 8, 9, 11, 12, and 17 were actually missiles used to test new types of engines, and some other vehicles were un-manned. Most X-planes are not expected to go into full-scale production; one exception was the Lockheed Martin X-35, which competed against the Boeing X-32 during the Joint Strike Fighter Program, and has entered production as the F-35 Lightning II.
List
In the list, the date is that of the first flight, or of cancellation if it never flew. ! class="unsortable" style="width:120px;" | Image ! Type ! Manufacturer ! Agency ! Date ! Role ! class="unsortable" | Notes
