X-planes

• 
  X-1
  (Manufacturer: Bell; first flight: January 19, 1946)
• 
  X-2
  (Bell; June 27, 1952)
• 
  X-3
  (Douglas; October 27, 1952)
• 
  X-4
  (Northrop; December 15, 1948)
• 
  X-5
  (Bell; June 20, 1951)
• 
  X-6
  (Convair), cancelled (1957)
• 
  X-7
  (Lockheed; April 26, 1951)
• 
  X-8
  (Aerojet; December 2, 1949)
• 
  X-9
  (Bell; April 1949)
• 
  X-10
  (North American; October 13, 1953)
• 
  X-11
  (Convair; June 11, 1957)
• 
  X-12
  (Convair; July 19, 1958)
• 
  X-13
  (Ryan; December 10, 1955)
• 
  X-14
  (Bell; February 19, 1957)
• 
  X-15
  (North American; June 8, 1959)
• 
  X-16
  (Bell), cancelled (1954)
• 
  X-17
  (Lockheed; April, 1956)
• 
  X-18
  (Hiller; November 24, 1959)
• 
  X-19
  (Curtiss-Wright; November, 1963)
• 
  X-20
  (Boeing), cancelled (1963)
• 
  X-21
  (Northrop; April 18, 1963)
• 
  X-22
  (Bell; March 17, 1966)
• 
  X-23
  (Martin; December 21, 1966)
• 
  X-24
  (Martin; August 1, 1973)
• 
  X-25
  (Benson; December 6, 1955)
• 
  X-26
  (Schweizer; July, 1967)
• 
  X-27
  (Lockheed), cancelled (1971)
• 
  X-28
  (Osprey; August 12, 1970)
• 
  X-29
  (Grumman; December 14, 1984)
• 
  X-30
  (Rockwell), cancelled (1993)
• 
  X-31
  (Rockwell; October 11, 1990)
• 
  X-32
  (Boeing; September 18, 2000)
• 
  X-33
  (Lockheed Martin), cancelled (2001)
• 
  X-34
  (Orbital Sciences), cancelled (2001)
• 
  X-35
  (Lockheed; October 24, 2000)
• 
  X-36
  (McDonnell Douglas / Boeing; May 17, 1997)
• 
  X-37
  (Boeing; April 7, 2006)
• 
  X-38
  (NASA; 1999)
• 
  X-39, unknown
• 
  X-40
  (Boeing; August 11, 1998)
• 
  X-41, unknown
• 
  X-42, unknown
• 
  X-43
  (Boeing; June 2, 2001)
• 
  X-44
  (Lockheed Martin), cancelled (2000)
• 
  X-45
  (Boeing; May 22, 2002)
• 
  X-46
  (Boeing), cancelled (2003)
• 
  X-47
  (Northrop Grumman; February 23, 2003)
• 
  X-48
  (Boeing; July 20, 2007)
• 
  X-50
  (Boeing; November 24, 2003)
• 
  X-51
  (Boeing; May 26, 2010)
• 
  X-52, skipped
• 
  X-53
  (Boeing; November, 2002)
• 
  X-54
  (Gulfstream)
• 
  X-55
  (Lockheed Martin; June 2, 2009)
• 
  X-56
  (Lockheed Martin; July 26, 2013)
• 
  X-57

Other

• 
  1958
• 
  1993
• 
  HiMAT
• 
  Hyper 3
• 
  wave rider
• 
  scaled composites
• 
• 
  XB-35
• 
  XB-70 (not strictly in the X-Planes series)
  (North American; September 21, 1964)
• 
  XB-70
• 
  air launched
• 
• 
• 
• 
  CT-41 French design
• 
  model testing