Sentry,
It depends on how you are going to display your model. If you build and display it as a complete model then I would only deal with external issues, the interior is not really visible once assembled. That being said, this is what I know is available.
1. Verlinden makes 2 sets for the "AM" B-25B. The first is a resin/pe detail set. It includes resin engines, seats, armor plating, ammo cases, ordinance, guns and other items. Pe includes intrument panel, ignition harness and turret details. Lots of good stuff for a reasonable price ($15.00 US)
Verlinden also makes a full set of resin flight controls that come in handy if you want to repostion them with less headaches ($12.00 US)
2. Aires makes a detail set that consists of the entire interior nose section of the aircraft from the bomb bay forward ($35.00 US), I would not spend my money on this.
3. Cutting Edge makes a set of resin cowlings (early style for the "B") that corrects all those issues ($18.99 US), a bit priecy if you ask me but still worth it. I modified the cowlings myself, it's easy.
4. Parts R Parts makes a resin update set that includes the correct exhaust stacks, engine fronts, cowl flaps and air scoops ($18.00 US) reasonably priced for what you get.
They also offer ammo, gas cans and incendiary rounds which were carried by Doolittle raiders ($9.00US)
5. Engines and Things offers resin R-2600 engines ($11.70US). Verlinden also offers a R-2600 engine and front w/pe set ($ ?)
I would go with the Verlinden detail set, Cutting Edge cowlings and Parts R parts update set at minimum. These will correct and improve most problem areas on this kit.
Check out this web site for details on many of the fixes I spoke of.
http://partsrparts.homestead.com/B25Bparts.html
Goodluck
Darren