The 737 wasn’t designed to be a fire bomber, nor was the C-130.
With the exception of the Canadair and Be-200 I'm hard pressed to think of any aircraft designed with water bombing in mind. Personally can't see what difference it makes as to the airframe used, the catalogue of aircraft used is long, Neptune, B-24, DC-7, Catalina, Electra, Avenger, Mars etc etc The 747 and MD-87 are now used as well.