The aircraft was unusually powered by three diesel engines, with one mounted high above the centerline driving a four-bladed propeller, and one on each wing driving three-bladed propellers. Also unusual was the aircraft's twin boom tail unit, and gun turrets at the bow and the stern of the fuselage, as well as behind the central engine. These features together produced the aircraft's ungainly appearance.
Various versions of the aircraft carried a variety of armarment including MG 151, MG 15, and MG 131 machine guns as well as various combinations of bombs and depth charges. There was also a minesweeper variant of the aircraft, which carried a degaussing device in the form of a large ring around the fuselage instead of weapons.
And since the BV138 could also take a lot of battle damage and keep flying, especially as the diesel fuel rarely ignited when hit by machine gun fire, she was generally well liked by her crews.
Although the BV138 was able to carry small loads of bombs and depth-charges and thereby do attack missions such as sub-hunting, most operations were pure reconnaisance and surveillance, often working together with the german U-boats.
The BV138 flying boats were used almost all over Europe and patrolled the North Sea, Skagerrack and Kattegatt, Baltic Sea, Arctic Ocean, Norwegian Sea, Bay of Biscay as well as the Black Sea and the Mediterranean.