On March 7, 1969 Apollo 9's lunar module, named Spider, made its first solo manned flight. It was the first space test of the complete system (command module and lunar module) and a critical milestone for the Apollo program.
Commander James McDivitt, Command Module Pilot David Scott and Lunar Module Pilot Russell "Rusty" Schweickart tested several critical tasks including rendezvous and docking of the command module with the lunar module.
Because of production problems the lunar module was not ready for the original Apollo 8 mission that would have used this crew. McDivitt still wanted the lunar module mission so his crew was swapped to become the Apollo 9 crew while the original Apollo 9 crew was bumped up to fly Apollo 8. Since it was NASA policy to keep the backup crew (normally scheduled to be a primary crew three missions later) with the primary crew, this also moved Neil Armstrong and his crew up to fly Apollo 11 instead of Pete Conrad's crew, who ended up flying on Apollo 12.