
I really don't see something like a B-25 being a problem. Whether it is marketable these days who knows?
I used to own many 1:6 vehicles and the White Scout car was in a very long box, huge by toy standards. Surely a B-25 would've fit in something as big with the wings detached.
If there is any chance of seeing any more 1:18 WWII fighters I still think the P-40E Warhawk and P-51B would be good. Staightforward paint jobs, no mottling to worry about, plenty of different choices.
Big four engine jobs? Not likely but on the other hand HK Models has made a 1:32 Flying Fortress, a couple of other WWII aircraft like the B-25. They cost a fortune unbuilt. Why not make a pre-assembled pre-painted version less complicated without as much detail, interior parts etc. A few companies like Easy Model base their planes on kits like Trumpeter. Corgi uses their Airfix kits for die cast from what I've read.