Mercury Seven
When the Chief of the Flight Crew Operations, Mercury Seven astronaut Deke Slayton, drew up a tentative schedule of Project Gemini flights, he assigned Mercury Seven astronaut Alan Shepard command of the first crewed flight, Gemini 3, with Next Nine astronaut Tom Stafford as his co-pilot. Mercury Seven astronaut Gus Grissom would command the backup crew, with Borman as his co-pilot. Under the crew rotation system Slayton devised, the backup crew of one mission would become the prime of the third mission after. Borman would therefore become the co-pilot of Gemini 6, which was planned as a long-duration fourteen-day mission.