This is a question that's on the Xbox IGN boards and it is an interesting question. What WAS the first FPS? So we are on the same page: I'm talking you control a player character PERSON (not a tank or space ship) in a 3D environment where you need a quick trigger finger.
Already games like Wolfenstein 3D and Catacombs of the Abyss were brought up, but I know there has to be something that came out before then. Phantom Slayer came to mind for the CoCo (1982), but does anyone else have any other ideas?