What about a 727 with tanks? Remove the Cooper Vane, and it's doable to open the rear door in flight... same with a DC-9 but the range isn't there.
No idea if there are any used ones on the market, but USCG HU-25s (Falcon 20) have a belly door that can be opened in-flight for making drops. Again though it's range-limited if you need 3000NM+
Last edited by MikeNYC; 3rd Aug 2012 at 14:26.