The G4M Medium Bomber (code named Betty by allied forces) was designed in 1939 for the Imperial Navy and went into production in 1941. It saw its first action in May of 1941 in China. Mitsubishi argued that the aircraft should be a four engine design but the Navy insisted on a twin engine design and long range.
The main design criteria for this aircraft was an operational range of 3,000 miles and a bomb load of 2000 lbs. Pilot armor, defensive armament, and protected fuel tanks were sacrificed for low weight, large wing fuel tanks, and extended range.
The G4M1 series was a twin engine design using two Mitsubishi 14 cylinder twin row Kasei-11 air cooled engines developing 1530 HP, giving the aircraft a maximum speed of 265 mph. The 1100 gallon fuel tanks gave the G4M1 an operational range of 3,100 miles.
The G4M2 saw the introduction of the water/methanol injection Kasei-22 engines which developed 1825 HP. This reduced the operational range to 2,980 miles but an increase in top speed to 271 mph.
The G4M3 variant had self sealing wing tanks, pilot armor, but this came at the expense of the fuel tanks being reduced to hold 968 gallons and a much reduced operational range of 2,350 miles. This design was released too late in the war to be an effective aircraft and only 60 of this designation were produced
The G4M1 was to play an important part in the sinking of the two British battle ships HMS Prince of Whales and HMS Repulse on December 10, 1941. Admiral Isoruku Yamomoto was intercepted and shot down by US P-38 fighters while flying in an G4M1 over Bouganville on April 18, 1943. A modified G4M2e version was built to be the launch vehicle for the MXY7 Ohka piloted rocket missile bomb. It was an G4M3 that carried the Japanese surrender delegation at war's end.
Over 2,400 G4M1 through G4M3 Medium Bombers were produced during the war.
The Betty bomber's main failing were the lack of pilot armor and the wing mounted fuel tanks were not self sealing therefore very venerable to destruction. So prevalent were the wing tanks to fire and explosion that US pilots nicknamed the Betty the "Flying Lighter".