Leviathan is a boss from 1941: Counter Attack. It is fought during mission 3 and again during mission 5.
The Leviathan is a large bomber with two engines. It has mounted turrets to shoot at the player but will also release balloons which block incoming fire but can be destroyed. The bomber must be taken out piece by piece before going for the cockpit.
Leviathan returns again later in the game with a enhanced design, now sporting two extra engines and proper wings, as well as more mounted turrets. It attacks in the same manner as the original except harder, as is taken out in the same manner.