There was a hand full of -10 and -30 freighters built, most of them quick change "combi's" with the windows in place. A few were built without the passenger windows, but emergency exit plugs had them. The few -10Fs and 30Fs built as pure cargo planes also did not have the rear air stair.
Most combi's were later bought up and the windows plugged over, only used for cargo service. The combi's did have a strengthened floor.
Navy / USAF C-9A's and C-9B's were offshoots of these commercial models with some differences.
DC-9 and DC-8 cargo doors had a similar design and had too many instances of coming unlocked in flight, and many FAA AD's issued trying to correct the problems. At least one DC-9-33F crashed when the door came completely open to the full up position after takeoff.