I was thinking along the lines of the Tigercat but with a fuselage like a Mosquito.
- Even the NA-40 prototype of the B-25 went almost 14,000lbs empty. Trying for 14,000lbs empty with a pair of R-2600s might take a bit of doing. You have about 3900lbs just for the bare(dry weight) engines. Even the starters and generators add weight.
- Planes listed had 3-4 man crews and pretty much hand aimed RCMGs so defensive armament isn't going to get much lighter.
- Point is there doesn't seem to be 3-4,000 lbs of empty weight (airframe) to spare OK n any of the small bombers that were made.
In a hypothetical situation where NA had no work I think they could have designed a metal mosquito and if it had Merlin engines probably about the same weight.