Here are a few pics showing the difference from the 'small' C47 with a huge cargo area and a 'large' B17 with a tiny cargo area.

C47 with paras

B17 fusalage where cargo would be loaded and paratroopers would be.

this is a version of the US 2 1/2 ton truck designed to be taken aparts and carried in a C47. It WOULD NOT fit in
a B24, B17, B25, nor B29 (unless you gut it of its bombbay permanently).

how you get in and out a B25, thru the belly. not especially healthy way for paratroops to drop or cargo to load/unload.

This is the only hatch to get into and out of a B17 by crew/passengers other than the belly hatch under the nose.
Yes you can certainly carry alot thru that door. Now match that up to pics of fully loaded paratroops struggling to get
thru the huge door on a C47.

loading 'real' cargo onto a C47. Now match that pic to the previous pic of the B17 door.

Loading troops into a C47. Now match that pic to the interior of the C47 and the B17.
Notes on the C87 version of the B24 heavy bomber as a purpose converted bomber....read it and weep:
The aircraft could also become unstable in flight if its center of gravity shifted due to improper cargo loading. This longitudinal instability arose from the aircraft's hasty conversion from bomber to cargo transport. Unlike a normal cargo transport, which was designed from the start with a contiguous cargo compartment with a safety margin for fore-and-aft loading variations, the bomb racks and bomb bays built into the B-24 design were fixed in position, greatly limiting the aircraft's ability to tolerate improper loading. This problem was exacerbated by wartime exigencies and the failure of USAAF Air Transport Command to instruct loadmasters in the C-87's peculiarities. The design's roots as a bomber are also considered culpable for frequently collapsing nosegear; its strength was adequate for an aircraft that dropped its payload in flight before landing on a well-maintained runway, but it proved marginal for an aircraft making repeated hard landings on rugged unimproved airstrips while heavily loaded.
The C-87 was rapidly displaced on the front lines by the Douglas C-54 Skymaster and Curtiss C-46 Commando, which offered similar performance combined with greater reliability and more benign flight characteristics. Some surviving C-87 aircraft were converted into VIP transports or flight crew trainers, and several others were sold to the Royal Air Force.
Again> This is why bombers are bombers and not transports and vice versa....