Designed to an incredibly demanding 1938 Navy specification, the G4M family (given the code-name "Betty" by the Allies) was the Imperial Japanese Navy's premier heavy bomber in World War II; yet the insistence on the great range of 2,000 nautical miles (3,706 km) with full bomb load made the saving of weight take priority over defence and the aircraft was highly vulnerable and not very popular. The wing was of the same Mitsubishi 118 section as the Zero-Sen and boldly designed as an integral fuel tank to accommodate no less than 5,000 litres (1,100 gal). The company kept recommending four engines and being overruled by the Navy, which, during the early flight-test stage, wasted more than a year, and 30 aircraft, in trying to make the design into the G6M bomber escort with a crew of ten and 19 guns.
Eventually the G4M1 was readied for service as a bomber and flew its first missions in South East China in May 1941. More than 250 operated in the Philippines and Malayan campaigns, but after the Solomons battle in August 1942 it began to be apparent that, once intercepted and hit, the unprotected bomber went up like a torch (hence the Allied nickname "one-shot lighter"). Total production reached the exceptional quantity of 2,479, most of them in the many sub-types of G4M2 with increased fuel capacity and power. Finally the trend of development was reversed with the G4M3 series with full protection and only 968 gal fuel.