Mods: feel free to edit/delete as needed to meet your preferred formatting:
See this link for the excellent reply to this question by u/Lord_mayor_of_reddit 5 yrs ago:
https://www.reddit.com/r/AskHistorians/comments/5toz0o/how_and_when_did_the_american_accent_come_to_be/?utm_source=share&utm_medium=ios_app&utm_name=iossmf