/r/linguistics might be better equipped to answer this.
More comments are welcome, but you might be interested in the following from the FAQ:
Would Americans at the time of the revolution have spoken with a distinct non-British accent?
When did the current dialect of American English diverge from traditional British English?
When did Americans lose their Brittish accent?
Differences in American and British English accents
At what point in time did Americans develop an American accent (or accents, for say the north and south)?