Zimmbous

It was some kind of flying (like airplanes) game for windows pc. Probably around the mid 90s, most likely windows 95 or late dos. All I remember is some kind of 3d graphics (might have been polygonal) and the concept of "flying fortresses" being important in some way (or maybe just the ultimate form of playable airplane or something). It might have been a sim game, but I remember combat being important. Something about the term "flying fortress" struck me as describing a flying base as opposed to its actual meaning (a B17 or whatever), since I didn't know it was a referential term at the time (so they might have been flying bases in the game). I remember some kind of brown or grey boxxy plane. That might have been the flying fortress?