The first manned flight of the Apollo CSM, the Apollo C category mission, was planned for the last quarter of 1966. Numerous problems with the Apollo Block I spacecraft resulted in a flight delay to February 1967. The crew of Virgil I. Grissom, Edward H. White II, and Roger B. Chaffee, was killed in a fire while testing their capsule on the pad on 27 January 1967, still weeks away from launch. The designation AS-204 was used by NASA for the flight at the time; the designation Apollo 1 was applied retroactively at the request of Grissom's widow.

Apollo 205, a second solo flight test of the Block I Apollo CSM, was planned but cancelled on December 22, 1966. The Schirra, Cunningham, Eisele crew from that flight became the backup crew to Apollo 204 (replacing the original backup crew of McDivitt, Scott, Schweickart). References: 26 , 27 , 33 , 60 .