In September 1967, Owen Maynard of the Manned Spacecraft Center in Houston, Texas proposed a series of Apollo missions that would lead up to a manned lunar landing. Seven mission types were outlined, each testing a specific set of components and tasks, and each previous step would need to be completed successfully before the next mission type could be undertaken. These were:

• A - Unmanned Saturn V and Command/Service Module (CSM) development (Apollo 4, Apollo 6)
• B - Unmanned Lunar Module (LM) development (Apollo 5)
• C - Manned CSM evaluation in low Earth orbit (Apollo 7)
• D - Manned CSM and LM development in low Earth orbit (originally planned for Apollo 8; flown as Apollo 9)
• E - Manned CSM and LM operations, a simulated lunar mission in an elliptical medium Earth orbit with an apogee of ), never flown