The PBY carried 4000lbs of bombs, depth charges or torpedoes, so that's actually slightly more than what the B-25 carries.
If Wiki's stats are to be believed that's actually MORE than the H8K (2x800kg torpedoes, or 1000kg of bombs or depth charges). However the H8K is slightly faster, has longer range, and is MUCH more heavily armed (5 defensive 20mm cannon!) However the PBY might be the more versatile aircraft, especially if we get a gun package option for a "Black Cat" with fixed forward guns like in the 25C.
JRM Mars might be interesting, although with only 7 built it probably wouldn't fit add criteria, although all of the production aircraft DID fly for the same squadron (VR-2).... Would basically be like a C-47 on steroids but make it supplies only (she could carry 32,000lbs of cargo).