To expand a little: The majority of early American settlers in the colonies were from England, so my assumption is that in the days of our founding fathers, most people spoke with a British accent. So when did the American accent branch off from the British, and how can we know this?
Here is a fairly comprehensive anwser.
Just a correction: perhaps a plurality of pre-U.S. Americans traced their roots back to England but almost certainly not a majority.
New York was a Dutch colony and most schools in the state were Dutch language until about 1800. Settlers from France, Germany, and Scandinavia, and many other places made up the majority of early settlers. Only a few were English.
New Jersey pretty much shared the same history of Dutch settlement in the early years, and Connecticut became home to many 2nd, 3rd, and 4th generation Dutch Americans too.
Similar stories can be found in other colonies. Delaware's earliest settlers were Swedish. South Carolina became home to a large population of French Huguenots. Pennsylvania was home to many German immigrants.
Add to that, "Great Britain" wasn't formed until 1707, and Engand, Scotland, Ireland, and Wales all had very distinct cultures and languages and accents when speaking f English, if they could even speak it. By that time, most of the original 13 Colonies were already decades old.
In other words, while many people assume that pre-Revolution America was made up of a critical mass of English immigrants, that was not true. By 1700, the pre-U.S. population was almost as diverse as any white suburban American town is today. By 1776, the majority of Americans were native born. There were already fourth, fifth, even sixth generation Americans who had no more connection to the Old World than native born Americans do today.
This doesn't answer your question but hopefully gives you better background on what you're asking.
Well language evolves over time, and the combination of the original English brought by these settlers combined with other countries' settlers' accents and possibly native tongues influenced American English differently from British English (which also evolved a lot, and in different directions due to different influences). Also, you have to look at the type of people coming to America, their English in some cases might have been different from that of some of the people staying in Britain, considering their demographic groups.