Looks lik a nice set. Not sure where the name comes from though, maybe a translation thing or following Academy's mis-naming. The aircraft is actually a CH-46 Sea Knight. It is called "Phrog" (due to its squat, amphibian-like appearance and tendency to bounce when taxiing) by crewmen, not Bull Frog.

The bullfrog is a name used for the CH-46E (lets call it late model E) when it got the larger fuel tanks. Early E's still had the smaller A,D,F sponson tanks. LINK Most people I knew just called it a 46. I am sure some might have used phrog but never heard bullfrog. One of those things that has a life of its own I guess.