I don't think anyone has mentioned the Air Mobility Museum at Dover AFB, Delaware, which has a great collection of "heavy metal" - tankers and transports of all sizes and other types besides. Also in the general area of Washington DC is the Glenn L. Martin Maryland Aviation Museum at Martin State Airport near Baltimore. There are good indoor displays about Martin aircraft, many of which were built there, and an outdoor collection further down the field, which is accessed with an escort from the museum staff.