Wolfenstein 3D is a first-person shooter video game developed by id Software and published by Apogee Software and FormGen. Originally released on May 5, 1992, for DOS, it was inspired by the 1981 Muse Software video game Castle Wolfenstein, and is the third installment in the Wolfenstein series. In Wolfenstein 3D, the player assumes the role of Allied spy William "B.J." Blazkowicz during World War II as he escapes from the Nazi German prison Castle Wolfenstein and carries out a series of crucial missions against the Nazis. The player traverses each of the game's levels to find an elevator to the next level or kill a final boss, fighting Nazi soldiers, dogs, and other enemies with knives and a variety of guns. Wolfenstein 3D was the second major independent release by id Software, after the Commander Keen series of episodes. In mid-1991, programmer John Carmack experimented with making a fast 3D game engine by restricting the gameplay and viewpoint to a single plane, producing Hovertank 3D and Catacomb 3-D as prototypes. After a design session prompted the company to shift from the family-friendly Keen to a more violent theme, programmer John Romero suggested remaking the 1981 stealth shooter Castle Wolfenstein as a fast-paced action game. He and designer Tom Hall designed the game, built on Carmack's engine, to be fast and violent, unlike other computer games on the market at the time. Wolfenstein 3D features artwork by Adrian Carmack and sound effects and music by Bobby Prince. The game was released through Apogee in two sets of three episodes under the shareware model, in which the first episode is released for free to drive interest in paying for the rest. An additional episode, Spear of Destiny, was released as a stand-alone retail title through FormGen. Wolfenstein 3D was a critical and commercial success and is considered one of the greatest video games ever made. It garnered numerous awards and sold over 250,000 copies by the end of 1995. It has been termed the "grandfather of 3D shooters", and is widely regarded as having helped popularize the first-person shooter genre and establishing the standard of fast-paced action and technical prowess for many subsequent games in the genre, as well as showcasing the viability of the shareware publishing model at the time. FormGen developed an additional two episodes for the game, while Apogee released a pack of over 800 fan-created levels. Id Software never returned to the series, but did license the engine to numerous other titles before releasing the source code for free in 1995, and multiple other games in the Wolfenstein series have been developed by other companies since 2001.