Bethesda Softworks started working on Fallout 3 in July 2004, but principal development did not begin until after The Elder Scrolls IV: Oblivion and its related extras and plug ins were completed. Bethesda Softworks decided to make Fallout 3 similar to the 前 two games, focusing on non-linear gameplay, story, and black comedy. Bethesda also chose to pursue an ESRB rating of M (for mature) によって including the adult themes, violence, and depravity characteristic of the Fallout series. They also decided to shy away from the self-referential gags of the game's predecessors that broke the illusion that the world of Fallout is real. Fallout 3 uses a version of the same Gamebryo engine as Oblivion, and was developed によって the team responsible for that game. Liam Neeson was cast as the voice of the player's father.
In February 2007, Bethesda stated that the game was "a fairly good ways away" from release, but that detailed information and previews would be available later in the year. Following a statement made によって Pete Hines that the team wanted to make the game a "multiple platform title", the game was announced によって Game Informer to be in development for Windows, Xbox 360 and PlayStation 3.
During a March 21, 2008 Official Xbox Magazine podcast interview, Todd Howard revealed that the game had expanded to nearly the same scope as Oblivion. There were originally at least 12 versions of the final cutscene, but with further development this expanded to over 200 possible permutations in the final release, all of which are determined によって the actions taken によって the player. Bethesda Softworks attended E3 2008 to showcase Fallout 3. The first live demo of the Xbox 360 version of the game was shown and demonstrated によって Todd Howard, taking place in downtown Washington, D.C. The demo showcased various weapons such as the Fat Man nuclear catapult, the V.A.T.S. system, the functions of the Pip-Boy 3000, as well as combat with several enemies. The demo concluded as the player neared the Brotherhood of Steel-controlled 五角形, 米国防総省 and was attacked によって an Enclave patrol.