I came across a site that's pretty interesting on facts and myths about Pitbulls. According to the Official Pitbull Site by Diane Jessup,

Hunting boar in the colonies. Primarily used as an animal which could pin and control large and dangerous beasts such as boar, bear and bull, dogfighting was never the original purpose of the bulldog. Dog fighters will argue this point, but the evidence is quite clear. The animals pictured here could enter the ring and win as representatives of the American pit bull today.
Pitbulls or the historical bull dogs (different from bulldogs) used to be bred for bull baiting. When this was banned, blood sport fanciers decided to make these dogs fight against each other.

If you wish to know more about the Pittie's history, just drop by this page: http://www.workingpitbull.com/history.htm