Call of Duty is a first-person shooter video game developed by Infinity Ward and published by Activision. It is the first installment in the Call of Duty franchise, released on October 29, 2003, for Microsoft Windows. The game simulates infantry and combined armswarfare of World War II using a modified version of the id Tech 3 engine. Much of its theme and gameplay is similar to the Medal of Honor series; however, Call of Duty showcases multiple viewpoints staged in the British, American, and Soviet theaters of World War II.
The game introduced a new take on AI-controlled allies who support the player during missions and react to situational changes during gameplay. This led to a greater emphasis on squad-based play as opposed to the "lone wolf" approach often portrayed in earlier first-person shooter games. Much of Infinity Ward's development team consisted of members who helped develop Medal of Honor: Allied Assault.
In September 2004, an expansion pack called Call of Duty: United Offensive, which was produced by Activision and developed by Gray Matter Interactive and Pi Studios, was released. An enhanced port of Call of Duty for the Xbox 360 and PlayStation 3, titled Call of Duty Classic, was released worldwide in November 2009 with the release of Call of Duty: Modern Warfare 2, being available via redemption codes included with the "Hardened" and "Prestige" editions of the game.