I do have a midwestern accent - typically -ing words end in an -ine sound; marry, merry and Mary all sound the same. It isn't as noticeable as a MN accent or a UP accent, but it is there. I lived in Boston for a while and everyone was particularly amused that I pronounced the word box in a very short way, and that I used the word bubbler in place of water fountain. And I once asked someone which "Bonnie" they meant, when in fact they had asked about "Barney". I had to ask them, "If you pronounce B-a-r-n-e-y as Bonnie, then how do you pronounce B-o-n-n-i-e?"
And yet I lived there long enough that when I'm not paying attention, words with an -ar sound often end up with an -ah sound instead. But I still pronounce marry, merry and Mary the same, so I never developed a true Boston accent!