well done, my friend!
My suspicion is that just as DB stood for Douglas Bomber, the DC stood for the Douglas Cargo version of this same aircraft. Had it advanced any further I believe that designation would have changed so as not to confuse it with the commercial DC aircraft. It does fit nicely into the general attempt by the AAC to use all/most of the existing multi-engine military aircraft as cargo carriers as well as bombers. The B-17, B-18 and B-24 were also seriously studied (and in some cases used) as both troop/personnel and cargo carriers as a secondary use to their capacity for bombing. This program began because as it became more and more likely a war was coming in Europe, combined with the general lack of cargo-carrying aircraft of any capacity, the bombers were the only aircraft with the capability to carry heavy amounts of personnel or cargo in a hurry. Otherwise, when a unit was transferred to another base the ground crews and equipment all had to sent via rail and/or truck. This would mean the bombers sat at their destination for many days essentially unable to perform their real task until the support personnel arrived via land. Fortunately, the DC-3 proved capable of being mass-produced, thus negating the need for their use as troop transports (although tests were run on both the B-17 and B-24 as late as 1944). However, the heavy bombers - especially the B-24 (quite apart from the C-87) - were used to carry cargo throughout the war and were studied for many such purposes, including carrying disassembled artillery pieces (105mm, etc.) to bases near the front lines.
I hope this helps!
AlanG
