Several planned missions of the Apollo crewed Moon landing program of the 1960s and 1970s were canceled, for reasons which included changes in technical direction, the Apollo 1 fire, the Apollo 13 incident, hardware delays, and budget limitations. After the landing by Apollo 12, Apollo 20, which would have been the final crewed mission to the Moon, was canceled to allow Skylab to launch as a "dry workshop" (assembled on the ground in an unused S-IVB Saturn IB second stage). The next two missions, Apollos 18 and 19, were later canceled after Apollo 13 and further budget cuts. Two Skylab missions also ended up being canceled. Two complete Saturn V rockets remained unused and were put on display in the United States.

In September 1962, NASA planned to make four crewed low-Earth-orbital test flights of partially equipped Block I Command/Service Modules (CSM) using the Saturn I launch vehicle, designated SA-11 through SA-14, in 1965 and 1966. However, the limited payload capacity of the Saturn I compared to the uprated Saturn IB would have severely limited the systems carried, and thus the testing value of these flights. Therefore, NASA canceled these flights in October 1963, and replaced them with two crewed Saturn IB missions, designated AS-204 and AS-205. These would be followed by the first uncrewed flight of the Lunar Module (LM) on AS-206, then the third crewed mission, designated AS-207/208, would use AS-207 to launch the crew in an improved Block II CSM, which would rendezvous and dock with the LM launched uncrewed on AS-208.

The crew selected on March 21, 1966, for AS-204 consisted of Command Pilot Virgil "Gus" Grissom, Senior Pilot Ed White, and Pilot Roger Chaffee, who named their mission Apollo 1. AS-205 was to be named Apollo 2, and AS-207/208 would be Apollo 3. The AS-205 crew were Wally Schirra, Donn Eisele and Walter Cunningham. However, AS-205 was later deemed unnecessary and officially canceled on December 22, 1966.

Schirra's crew then became the backup for Grissom's crew, and the crewed LM mission became the second crewed mission, redesignated AS-205/208 and crewed by the original backup for Grissom's crew: Command Pilot Jim McDivitt, CSM Pilot David Scott and LM Pilot Rusty Schweickart. They immediately began their training in the first Block II Command Module CM-101, as Grissom's crew were preparing for a February 1967 launch.

Then, on January 27, 1967, Grissom, White, and Chaffee were killed in a flash fire in their spacecraft cabin during a test on the launch pad, interrupting the program for 19 months to identify and fix the root causes of numerous safety problems. This forced cancellation of plans to fly any crewed Block I spacecraft, and effectively forced a "reboot" of all crewed mission plans.

In September 1967, NASA created a list of remaining mission types necessary to achieve the first crewed lunar landing, each designated by a letter A through G, where G would be the first crewed landing. This list was later extended through letter J to cover follow-on lunar missions.

Two uncrewed Saturn V test launches (A missions) were flown as Apollo 4 and Apollo 6. A third test was planned but canceled as unnecessary.

The first development Lunar Module, LM-1 was flown uncrewed (B mission) as Apollo 5. A second uncrewed test was planned using LM-2 but was canceled as unnecessary. LM-2 was retrofitted to look like a production LM which would land men on the Moon and was donated to the Smithsonian National Air and Space Museum, where it is currently on display as a simulation of the Apollo 11 first landing.