The strealth bomber is definitely underpowered in Civ IV. The B-2 Spirit is not completely invisible, Reaper, but it has radar-defeating technology and has the radar crossection of a bird. This, combined with a black paint job and flying at night makes the plane "virtually" invisible. I personally hated having "Fighters" (not Jet Fighters) intercept mine. A B-2 flys at above 60,000ft, over 600mph and at night. Typically WWII piston fighters like the Spitfire, FW 190, or Mustang flew below 40,000 ft, under 450mph and in the daytime. The two planes would never have met in the same sky, let alone an intercept taking place. Like a fight between a shark and a grizzly bear.
Also, a Stealth Bomber ought to have a strength of about 50. It has the bombing power of whole Groups of WWII bombers. (Bomber Groups)