Sonic Adventure was only released in Japan at first. In 1999, the game was re-released, but now worldwide (and in Japan with the title Sonic Adventure International). The revised edition includes two voice tracks (Japanese and English) and subtitles for different languages. The Internet Ranking Mode was opened up worldwide, the camera angles were fixed, voice and environmental effects' volume was increased and there were minor improvements to the minigames.
The game took 1 1/2 years to plan out and to create the concept
Sonic X-Treme, an unfinished Sonic game in 3D, was originally to be released for the Sega Saturn in 1996. However, with so many delays, and without the help of Yuji Naka (was working on NiGHTS into Dreams at the time) the game got canned. Sonic X-Treme was originally going to be the first Sonic 3D game to come out. Instead, Sonic Adventure took the lead of that for the Dreamcast.
Many of the textures used in the game were actually photos taken by Sonic Team on their trip to Central America
At the Sonic Adventure conference in 1998, pamphlets were passed out advertising it. One of the pictures showed a screenshot from the first Sky Chase minigame, which included a dragon attacking the Tornado. This dragon never made it into Sky Chase (nor anywhere else) in the final game. The dragon did in fact make it into another Sonic game. In 2003 it was used as the final boss for "Sonic Heroes".
Many, MANY scrapped ideas from previous Sonic games were put into this game. (eg The boulder, object interaction, etc...)