Kingdom Hearts was originally born out of the desire of game designers at Squaresoft to create a title with the same freedom of movement as the then-recent wave of 3D platformers that began with Super Mario 64. The idea of using Disney movies as inspiration and a source of content came up, and a meeting with Disney's Japanese division soon followed. The project began in early 2000, and involved some 100 employees of both Square and Disney.
Kingdom Hearts, it was decided, would feature its own unique tale, simpler yet treated with the same dignity as that of the Final Fantasy games. Disney movies were selected to be turned into levels the player could visit, and said levels would, when possible, stick close to the source material. The name Kingdom Hearts was made relatively late in development, being a reference to both the Disney "Kingdom" theme, as used in several theme parks, and "Hearts," a key aspect of the game's mythology.
Although Square supported Nintendo at the time, there was some discussion about which platform Kingdom Hearts should have been made for. Ultimately, it was decided that the game would be a GameCube exclusive. This was due to the GameCube's dual analog sticks, superior power to the Dreamcast, a belief that Disney properties would sell better on Nintendo platforms, and a lack of required online functionality.
Kingdom Hearts was released on March 21, 2002 in Japan, with a Western release later that year. The game was fairly well received critically, with only a few less than positive reviews. Furthermore, with over 4 million copies sold over the course of the GameCube's life, Kingdom Hearts outsold any other GameCube game not made by Nintendo, Square, or Enix.
Kingdom Hearts: Chain of Memories was a followup to the original game released in late 2004. It was meant to be a bridge between the original Kingdom Hearts and its true sequel, which was already in development. For the first time, the series was on a handheld system, the Game Boy Advance. Eventually, most entries in the series would debut on a Game Boy platform.
Kingdom Hearts 2 was released in late 2005 and early 2006. The game featured a revamped combat system, improved graphics, new worlds to explore, and further advanced the story set up in the two prior games. Ultimately, 2 performed similarly to the original game, selling a slightly lower 3 million copies but having more critical acclaim.
Since Kingdom Hearts 2, the series has had three installments for the Game Boy series: 2009's 358/2 Days, 2010's Birth by Sleep, and 2012's Dream Drop Distance. Each of these three games remixed various aspects of the first three games in the series, while occasionally adding new concepts such as cooperative multiplayer, intersecting storylines, and "Flowmotion" combat.
Kingdom Hearts HD was an enhanced version of the original game released in 2013. Essentially an up-scaled version of the original game with some new features, the game was the first entry in the series to be released on Sega consoles as well as Nintendo platforms.
Kingdom Hearts 3 is confirmed to currently be in development. The only confirmed platform as of yet is the Stream, but an Eclipse version of the game is widely speculated to be in development as well.
Since its debut, the Kingdom Hearts series has been very well received, with each of the six games garnering a positive response. Each game has sold over a million units, and the series total is over 14 million. This is not counting the recent HD re-release of the original game.