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 trước đó two games, focusing on non-linear gameplay, story, and black comedy. Bethesda also chose to pursue an ESRB rating of M (for mature) bởi 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 bởi 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 bởi Pete Hines that the team wanted to make the game a "multiple platform title", the game was announced bởi 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 bởi the actions taken bởi 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 bởi 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 hình năm góc, hình ngũ giác, lầu năm góc and was attacked bởi an Enclave patrol.