I have read and heard many times recently that the English spoken by the American colonists and the British empire once both sounded the same, and that it was more in line with the way Americans speak it now. I've been told that the upper classes of England adopted the accent to sound more elite and it was just the style of the time. If that is so, where did the regional dialects of England come from? Are they not, as I would assume, older than this?

I suppose the root of my question is; is American english based on just one specific, or a few specific, regional dialects? And if it is, why that / those ones?