Lockheed XF-90
Lockheed XF-90 on display in the National Museum of the U.S. Air Force Cold War Gallery. A cargo version of the B-29, The C-97 Stratofreighter first flew inThe X-15 was an important tool for developing spaceflight in the 1960s, and pilots flying above 50 miles altitude in the X-15 earned astronaut wings. (U.S. Air Force photo by Ty Greenlees)