I have maintained for years that I have always had no accent, but come close to the generic American standard. My mother had a bit of a NH accent, and my grandfather on the other side had a strong Canadian southern Maritimes accent (Yarmouth County, NS) that is very similar to a Maine/Mass north shore accent. But I have none.
Well, I have none now. But I just watched a video of myself on 12/14/69, when I was a junior in highschool, and can hear a slight NH accent on the broadened "a's." I must not have lost that until I went to college out of New England.
The things we know we don't always know.