As another poster pointed out the other day, the typical Southern accent is an outgrowth of the Scots-Irish dialect[s] of English. It is actually much closer phonetically to the spoken language of Shakespeare than is the broad Midwestern accent which this lady, in her ignorance, apparently regards as the standard.

This can be seen in the relative ease with which actors transition from one to the other. Think of Vivien Leigh in Gone with the Wind. It also works the other way 'round.
A striking example would be John Hillerman, a native of Dennison Texas who played the upright English major-domo Higgins on Magnum PI. Hillerman sometimes played Higgins' Texan cousin Jim Bob during the series. Jim Bob's Texas drawl is in fact Hillerman's natural speach, while Higgins' pseudo-Oxonian is a put-on."

A major, if not predominant, factor in much of the Midwest is the large German influence in the region, combined with Dutch, Swiss, Alsatians, and Austrians. People of German and other Central European descent are the largest single ethnic group in a wide swath of the country stretching from south central Pennsylvania westward to Montana and western Nebraska. People of British descent are in the majority mainly in the southernmost counties of Ohio, Indiana, and Illinois, as well as much of Missouri and Kansas. (Most of these areas were settled by Virginians, Kentuckians, and other Southerners.) Minnesota, parts of Wisconsin, and the Upper Peninsula of Michigan have a strong influence of Scandinavians. The old Rust Belt cities from Pittsburgh to Milwaukee, as well as St. Louis and Kansas City, also have high concentrations of white, non-German Catholics, much like their East Coast counterparts from Portland, Maine, to Baltimore. Chicago and Detroit also have large Eastern European Jewish settlements.
I would include in the overall Midwest definition those parts of Montana, Wyoming, and Colorado east of the Front Range, as well as western Pennsylvania and northern West Virginia due to the similarities in economy, population, and language to their Midwestern neighbors. I would exclude Oklahoma due to the predominantly Southern influence on that state.

Certain Midwestern distinctives, like the hard "r", may reflect the Germanic roots of much of the region's population. Regarding Gone With the Wind, it is worthy to note that Clark Gable, a native of Cadiz, Ohio, and partially of German ancestry, did not even attempt to sound Southern. During the Golden Age of movies, a large number of actors from mid-America did not try to sound Southern, even when their roles called for a drawl: John Wayne (Iowa and California); James Stewart (western Pennsylvania); Gary Cooper (Montana). OTOH, in the recent movie Cold Mountain, only one of the lead actors (Renee Zellweger) was from the South. Nicole Kidman (Australian), Donald Sutherland (Nova Scotian), and Jude Law (English) all delivered competent Southern accents.