After Armstrong served as backup commander for Apollo 8, Slayton offered him the post of commander of Apollo 11 on December 23, 1968, as Apollo 8 orbited the Moon.
Tras servir como comandante de reserva do Apollo 8, Slayton ofreceulle o posto de comandante do Apollo 11 o 23 de decembro de 1968, despois de que o Apollo 8 orbitase a Lúa.