Actually started on spitfires a British plane but hey, America invented everything first even though they didn't right?! Edit: after googling this to make sure I was right, it was indeed British fighter pilots who did it although the plane was a warhawk not a spitfire. Spitfires did eventually get painted with the shark mouth but they weren't the first.

But it continued as an American tradition in every war since many Americans paint their helis and jet with this emblem compared to the RAF that stopped using it for the most part so it may not be originally American but it still is a very american thing.