The tail gunner of a Japanese Mitsubishi G4M2 “Betty” bomber is equipped with a Type 99 20mm cannon.

Rear armament of the G4M2

Imperial Japanese Navy Air Service Pilots with a Nakajima J1N1 Gekkō ("Moonlight") twin-engine aircraft used for reconnaissance, as night fighter, and for kamikaze missions. Second from the left – the famous Satio Endo, - who shot down 8 and damaged 8 American Boeing B-29 Superfortress bombers; Circa 1943-44.

Imperial Japanese Navy Pilots

The Mitsubishi G4M was a twin-engine, land-based medium bomber operated by the Imperial Japanese Navy from 1940 to 1945.  The Allied reporting name was "Betty".