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.
Below is a listing with relevent documents for each.
The X-5 (Bell Aircraft)
The X-20 "Dyna-Soar" (Boeing)
The X-20 Dyna-Soar ("Dynamic Soarer") was a United States Air Force (USAF) program to develop a spaceplane that could be used for a variety of military missions, including reconnaissance, bombing, space rescue, satellite maintenance, and sabotage of enemy satellites. The program ran from 24 October 1957-10 December 1963, cost US$660 million, and was canceled just after spacecraft construction had begun.