Development of the "Ginga" started in 1940 at the Yokosuka Arsenal, design by Tadanao Mitsuzi and Masao Yamana the P1Y "Ginga" would prove to be the best Japanese twin engined bomber of the war. The first Y-20 prototype took to the air in August 1943, but it did not enter service up till the end October 1944 and saw no combat until the Okinawan campaign in 1945, this was due to horrendous maintenance problems and very unreliable engines.
The third prototype was used as a testbed to test the Tsu-11 Campini type motor-jet engine, intended for the Ohka Model 22. The Tsu-11 engine was fitted under the fuselage belly just behind the bomb bay.
The P1Y3 "Ginga" Model 33, was intended to be used as a mother ship for the Ohka 22, which would be fitted in the bomb bay. To achieve this the aircraft had a enlarged fuselage, extended wings (72' 2'') and the more powerful Nakajima Homare 21 radial engines.