The Voice of America has a website for ESLers . They use American Broadcast English, more or less, which is based on the Midwest accent.
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CharmYouso the general American accent is basically based on Midwest accent?Yes.
CharmYoushould I say Midwest accent or Midwestern accent?Either way, but there are not two forms for, say, "East-coast accent", so "Midwest" is probably better.