What breeds make up an American bulldog?

There are a few different theories on the origin of the American Bulldog. One states that the current American Bulldog is a direct descendant of English bulldogs brought into the US colonies. They say the American is in fact just the English which was selectively bred for work over the years.

Another, more realistic theory is that Bull Terriers and Bulldogs were chose selctively to create the American Bulldog. As pedigrees were not typically kept early on, it is hard to determine what was added in here and there to enhance certain traits.