Thirteen flight-quality Gemini spacecraft were built, but only twelve were launched into space. The thirteenth was spacecraft 3A, which began life as a boilerplate vehicle. Early in 1963 NASA decided it needed a replacement for spacecraft 3, which would be used for the first Gemini manned flight rather than altitude chamber tests. 3A was employed in hatch-opening tests and then was rebuilt as a flight-qualified vehicle, lacking only a heat shield and flight ejection seats. It was subjected to the extreme thermal and vacuum conditions of space in the McDonnell altitude chamber from December 19, 1964, to February 19, 1965, in a test series called Project Orbit. Spacecraft 3A underwent three test runs simulating over 220 orbits, resulting in the modification of the spacecraft coolant system.

The Smithsonian received 3A from McDonnell in 1971 as part of a shipment of surplus Gemini hardware from the military Manned Orbiting Laboratory program.