The design that emerged from this study was for an all-metal mid-wing monoplane with fabric-covered control surfaces and powered by four 1,870 hp Nakajima NK7A Mamoru 11 air-cooled radial engines driving four-bladed propellers. Notable features included a long ventral bomb-bay, glazed nose and twin tailfins replacing the DC-4E's distinctive triple rudder. The DC-4E's retractable tricycle undercarriage was retained, as well as the original wing form and powerplant arrangement. Defensive armament comprised one 20mm Type 99 Model 1 cannon each in a power-operated dorsal and tail turret plus single-mount hand-operated 7.7mm Type 97 machine guns in the nose, ventral and beam positions.
The first prototype G5N1 made its maiden flight on 10 April 1941. Overall performance proved disappointingly poor however, due to a combination of excessive weight, the unreliablity of the Mamoru engines and the complexity of the design. Only three more prototypes were completed. In an attempt to salvage the project, two additional airframes were fitted with 1,530 hp Mitsubishi MK4B 12 "Kasei" engines and redesignated G5N2s. Although the Mitsubishi engines were more reliable than the original Mamoru 11s, the aircraft was now even more hopelessly underpowered and further development of the type was halted.
Of the six completed Shinzans, four of them (two G5N2s and two G5N1s re-engined with the Kasei 12) were relegated for use as long-range Navy transports under the designation Shinzan-Kai Model 12 Transport G5N2-L. The Allies allocated the code-name "Liz" to the aircraft, in the expectation it would be used as a bomber.
G5N1: Four-engined heavy bomber. Production version, four built.
G5N2: Four Mitsubishi MK4B 12 "Kasei" radial engines in place of Nakajima Mamoru 11 engines. Two built.
G5N2-L: Long-range Navy transport conversion.
Nakajima Ki-68: Japanese Army heavy bomber. Four Mitsubishi Ha-101 or Nakajima Ha-103 engines.
Kawanishi Ki-85: Japanese Army heavy bomber. Four Mitsubishi Ha-111M engines.