X-1	Supersonic flight	1946 - 1958	19
X-1A	Supersonic flight	1946 - 1958	5
X-1B	Supersonic flight	1946 - 1958	6
X-1E	Supersonic flight	1946 - 1958	10
X-2	Supersonic swept wing	1955 - 1956	6
X-3	Sustained flight at Mach 2+, low aspect ratio wings, titanium construction	1953 - 1956	9
X-4	Semi-tailless aircraft	1954	9
X-5	Variable sweep wings	1952 - 1955	9
X-15	Hypersonic flight at high altitude	1959 - 1968	39
X-24	Lifting body flight research	1969 - 1975	17
X-29	Forward Swept Wing	1984 - 1992	11
X-31	Enhanced Fighter Maneuverability Demonstrator	1992 - 1995	24
X-33	Advanced Technology Demonstrator	1996 - present	9
X-36	Tailless Aircraft Research	1996 - present	10
X-38	Experimental Crew Return Vehicle	1996 - present	18
X-Wing	Rotor System Research Aircraft	1986 - 1988	1
XB-70A Valkyrie	Stability and handling of large delta-wing aircraft at high speed	1967 - 1969	5
XF-91 Thunderceptor	???	early 1950s	1
XF-92A	Evaluate delta wing concept	1953	8
XV-15 Tilt Rotor Aircraft	Improve design and efficiency of rotorcraft using tilt rotors	1980 - 1981	1

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