While the bomb bay would be used to hold the proposed conventional drop ordnance, the aircraft itself would be defensed through traditional ballistic weapons - machine guns and automatic cannons. This included 1 x 7.7mm Type 92 machine gun on a trainable mounting at the nose, another at a rear-facing ventral position, and one machine gun side-facing at each waist position. Beyond this was to be 1 x 20mm Type 99 Model 1 automatic cannon installed at the nose in a power-operated dorsal turret and similar armament installed at the tail in its own powered emplacement. All told, the bomber would have been well-defended through this network of guns.
The G5N, in its initial form as "G5N1" (and further classified as "Type 13 Land-Based Attack Bomber "Shinzan"), achieved its first-flight on April 8th, 1941 but the heavy design, coupled with the underperforming engines, left much to be desired of the large bomber (just two were built to the original standard). Engineers attempted to rectify the issues and three additional prototypes eventually joined the development phase bringing the airframe count to four. At one point, another pair of aircraft were added to the mix and re-engined to take 4 x Mitsubishi MK4B 12 "Kasei" radials of 1,530 horsepower - resulting in the switch to the "G5N2" test production designation. By this point, the war had engulfed Japan industry completely and aero-related resources were allocated to the ongoing war effort, leaving little room for development of a whole new bomber like the G5N.
So not all would be lost, of the six available airframes, four were reconstituted as long-range transports by the IJN and served as "G5N2-L" (further named "Shinzan-Kai"). These were identified by American military observers and afforded the codename of "Liz" for their time in service.
The Nakajima "Ki-68" was another bomber form proposed to the IJA based on the G5N1 design and were to field 4 x Mitsubishi or Nakajima powerplants. Plans for this version were shelved before the end of 1941. Another proposed IJA bomber form, the "Ki-85", was to carry 4 x Mitsubishi Ha-111M engines and its full-scale mockup was, indeed, completed in 1942 though this project also came to a halt and was cancelled in full in May of 1943.
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