Call of Duty is a 2003 first-person shooter game developed by Infinity Ward and published by Activision. It is the first installment in the Call of Duty franchise,^[3] released on October 29, 2003, for Microsoft Windows. The game simulates infantry and combined arms warfare 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 American, British, and Soviet campaigns of World War II in Europe.

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. The game received critical acclaim and won several Game of the Year awards from reviewers.

In September 2004, an expansion pack called Call of Duty: United Offensive, which was produced by Activision and developed by Gray Matter Studios and Pi Studios, was released. At the same time the N-Gage Version got an Arena Pack with 3 new Levels.^[4] An enhanced port of Call of Duty for the PlayStation 3 and Xbox 360, titled Call of Duty: Classic, developed by Aspyr, 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.^[5]

As a first-person shooter, Call of Duty places the player in control of an infantry soldier who makes use of various authentic World War II firearms in combat. Each mission features a series of objectives that are marked on the heads-up display's compass; the player must complete all objectives to advance to the next mission. The player can save and load at any time, rather than the checkpoint system utilized in later Call of Duty games.

The player has two primary weapon slots, a handgun slot, and can carry up to ten grenades. Weapons may be exchanged with those found on the battlefield dropped by dead soldiers. Unlike later Call of Duty games, the first allows the player to toggle between different firing modes (single shot or automatic fire). Call of Duty was one of the early first-person shooters to feature iron sights in game play; by pressing the corresponding key the player aims down the gun's actual sights for increased accuracy. In addition to weapons carried by the player, mounted machine guns and other fixed weapon emplacements are controllable by the player.

The game uses a standard health points system, with a limited amount of health reflected by a health bar. Medkits scattered throughout the levels or dropped by some foes are used to restore health when the player is injured.

Call of Duty also featured "shellshock" (not to be confused with the psychological condition of the same name): when there is an explosion near the player, he momentarily experiences simulated tinnitus, appropriate sound "muffling" effects, blurred vision, and also results in the player slowing down, unable to sprint.

As the focus of the game is on simulation of the actual battlefield, the gameplay differed from many single-player shooters of the time. The player moves in conjunction with allied soldiers rather than alone; allied soldiers will assist the player in defeating enemy soldiers and advancing; however, the player is given charge of completing certain objectives. The game places heavy emphasis on usage of cover, suppressive fire, and grenades. AI-controlled soldiers will take cover behind walls, barricades, and other obstacles when available.

Call of Duty was developed by Infinity Ward, a new studio formed in 2002 originally consisting of 21 employees, many of whom were project lead developers of the successful Medal of Honor: Allied Assault released the same year. Led by Chief Creative Officer Vince Zampella, development began in April 2002, and the team grew to 27 members by May 2003. At its beginning, the project was nicknamed "Medal of Honor Killer".^[6] Using an enhanced version of the id Tech 3 game engine developed for Quake III Arena and an in-house skeletal animation system called "Ares", Infinity Ward set out to develop a new World War II-era video game that, unlike many of its predecessors, placed more emphasis on squad-based play with intelligent assistance from teammates during large-scale battles. The team also extensively researched weapons, artillery, and vehicles from World War II to enhance the authenticity of animation and sounds used throughout the game.^[7] The game's budget was $4.5 million.^[8]

Another area the development team focused on was their artificial intelligence (AI) pathfinding component dubbed "Conduit". The ability to suppress the enemy with cover fire and clear obstacles, such as fences and windows, was tightly integrated into the squad-based aspect of the single-player campaigns. The AI in the game was designed to flank the opponent, bank grenades, and move from one cover point to another.^[1] Lead animation director Michael Boon explained that actions which would have normally been scripted in past games were moved to a dynamic AI environment, in order to help create a different experience each time levels are replayed.^[9] While the campaigns were the primary focus, development of the multiplayer modes were tailored to please modders. Zied Rieke, a lead designer, clarified that gameplay and modes were written in script making it "extremely easy for players to make their own modifications to Call of Duty multiplayer".^[9]

Call of Duty spawned numerous spin-offs and sequels, as part of the Call of Duty series. Its expansion pack - Call of Duty: United Offensive was developed by Gray Matter Studios and released September 14, 2004. Call of Duty 2 was also developed by Infinity Ward and was released in October 2005. Some Call of Duty spinoffs were developed for consoles, such as Call of Duty: Finest Hour by Spark Unlimited and Call of Duty 2: Big Red One by Gray Matter Studios (merged into Treyarch). The franchise eventually had over fifteen more sequels and spin offs.

Call of Duty Classic is a downloadable version of Call of Duty for Xbox 360 and PlayStation 3, featuring HD resolutions.^[31] Tokens to download the game ahead of its release were sold along with special "Hardened" and "Prestige" editions of Call of Duty: Modern Warfare 2,^[31] and the game was publicly released on December 2, 2009.

IGN rated this version 7.5, citing it as not well adapted to the consoles, as well as criticizing the multiplayer for only supporting up to eight players.^[32]

The music for the game was created by Michael Giacchino and was originally released in 2003 as the Call Of Duty Official Soundtrack Sampler CD, part of the game's pre-order from EB Games in Europe. The music was later released in the United States in 2005 as part of the Deluxe Edition, which included Call of Duty: United Offensive.^[33][34]