Found this on another form. Not modeling related but, it gives a rational for the individual exhaust set of some B-25s.
"Those bumps are very definitely fairings over individual exhaust stacks, which is the correct configuraton for a war-time B-25. Most of the Mtichells flying today have been converted to the later TB-25 configuration, which uses a collector ring and single exhaust stack outboard, but they still retain the individual stacks on the inboard side. A lot of this had to do with getting carburetor heat, as the wartime birds simply opened a scoop on the top of the nacelle behind the two top stacks. This sure ducted warm air into the carbs, but it was a little shy on oxygen, so the resultant power loss was enormous. With the collector ring and the stack, carb heat could be obtained in a more traditional way with a muff over the exhaust pipe to heat ambient air."
Which raises the question of the inboard single exhaust fairings placement on some trainers. I've included a photo of Miss Mitchell. I'll stop stealing your thunder (research) George.
1 attached image. Click to enlarge.