In 1934 Cadet Ushakov of the Soviet Naval Academy presented an engineering plan to build a plane that would land on the water and submerge, armed with torpedoes. The Soviet government decided that his plan deserved attention and funded an attempt at creating a prototype of the LPL or flying submarine. The idea was to have a plane that was powered by three propeller driven engines, and then two electrically operated underwater engines. This plane would have a three man crew, a range of about 500 miles and the ability to fly low and at night to infiltrate behind mine traps in US military harbors, deliver two 18 inch torpedoes and then egress.
Source: Dark Roasted Blend, Wiki