I-93 to the ski slopes of Vermont in the north. However, for better or worse, our forefathers filled in the Back Bay and built the MassPike, which you may be surprised to learn connects into I-90 and runs west all the way at least Chicago. Weird, but true.
Truth is, no can say for sure.
The Midwest is a state of mind, a somnolent state of mind. For that reason I've always put the border at about Framingham. Framingham is where the Irish and Italians start to run out, replaced by the lingering old Yankees and ethnically-neutered suburbanites transplanted from across the country to work in suburban office parks and shop in large chain stores just like the ones they left behind. The suburbanites are a lot like the Yuppies moving into Southie these days, only older and less pretentious. Worcester is the Midwestern town most like Boston, only less so.