That's essentially what I meant. However I take back what I said; I thought the ITA software would only give you a 200 or 300mi radius, but it will actually go up to 2,000 so in reality you could search the entire US. Just pick something very central like ORD, MCI, DFW or STL. The problem here is that since the US is much longer than it is wide, you'll end up with airports in Canada and/or Mexico and/or the Caribbean and/or Central America as well. Filtering them will be a lengthy process since the ITA software includes many Podunk airports without commercial service and you'll have to look them up to know if they're in the US or not, since they're not regularly mentioned here.

If you are going to attempt this, my suggestion would be to do 3 different 500mi radius searches, perhaps originating at LAS, MCI and PIT so that you'll be able to span the US but rarely if ever exceed its borders. You can do some filler searches for the westernmost points in the US as well as parts of the deep south cut off by the MCI search.