The F-111 was originally (believe it or not) developed as a fighter-interceptor and ended up evolving into an attack aircraft and electronic warfare platform (EF-111 Raven). It was one of the last of the "Century Series" to be developed before the DoD changed the numbering system in the early 1960s and we started over again, ending up with the F-4, F-5, F-15, etc.
Basically, "F" is fighter, "B" is bomber, "A" is attack, "C" is cargo, "K" is tanker, "H" is helicopter. There's other letter prefixes that add up to a dizzying array of combinations.
}:-)4
Well, of late, the "F" designation has been "improperly" used to attract the best pilots, who are loath to fly attack/bombers. e.g. the F-17