Mentioning previous answers here is not intended to discourage further questions. Especially because many of the previous questions I've seen so far have been about "the American accent" and "the British accent", which sometimes gets the response that there's never been a "the" on either side of the equation. Or more generally consider American versus British rather than different Americans versus British. But several do mention an important point in this, "Immigrants brought with them their own accents and dialects and used them once they arrived in the US."

In the FAQ about this:

• What caused Americans to develop a different accent from the British? featuring /u/keyilan.
• How has the American Southern accent changed since the Civil War? Before the war, was it perceived as prestigious in the North and elsewhere? by /u/tropical_chancer

In search hits for "American accent", the best for you appears to be How and when did the "American" accent come to be?, the two parts by /u/lord_mayor_of_reddit. They have a lot of detail, like an example of a distinct Dutch-influenced accent in New York in 1687, and a visitor in 1724 mentioning different American accents.

They wrote a little more in Who was the first President of the United States to speak with an "American" (not recognizably British) accent?.