The Block I spacecraft was the first version, used for testing the heat shield and other systems. The first manned flight of a Block I was to be Apollo 1, a Saturn IB flight. The pad fire and resulting disaster revealed design flaws in the construction of the spacecraft. After the Fire, the manufacturer, North American Rockwell Started work on the Block II, they did not charge NASA any money for the improvements they made that may have been attributed to the fire. A block II modified Block I first flew with Apollo 4.

Manned flights of the Apollo Project only used Block II Command Modules after the Apollo 1 tragedy.