The X-Planes conjure up images of Chuck Yeager first breaking the sound barrier in the X-1 or of the black, bullet-shaped X-15 streaking through the skies at over Mach 6. But there have been 48 more X-designations assigned, as well as a host of other experimental aircraft that did not fall into the formal X-series. Some of these investigated very low-speed flight, vertical take-off, or were technology demonstrators for production military aircraft. Others were missiles, or gliders. One was supposed to be a spacecraft. This book does not limit itself to the formal X-series of aircraft, but instead covers many of the experimental aircraft used by the U.S. military and NASA for flight research.