The G5N Shinzan (Mountain Recess) was the first attempt by Japan to design and build a land-based four-engined bomber equal to those fielded by its enemies — particularly the United States. It was both the first land-based four-engined aircraft ever designed for the Japanese Navy (the Army largely ignored four-engined bombers until much later in the Pacific War period) and the first Japanese aircraft ever to utilize a retractable nosewheel-type (i.e., tricycle) undercarriage.

In 1938, the Japanese Naval Staff began to evince an interest in acquiring a land-based attack bomber with a range of up to 4,000 statute miles — far greater than the range requirement of the Mitsubishi G4M twin-engined bomber then in the design stage. Although previously the Naval Staff had resisted the creation of four-engined bombers, and had indeed insisted on the G4M being designed as a twin-engined aircraft despite the designer’s preference for four engines, it was plain that the new range requirement would necessitate a four-engined configuration. Japan lacked sufficient experience in designing and building such large aircraft, and so the Navy made arrangements for Japan Air Lines to purchase the sole prototype of the American Douglas DC-4E four-engined transport. Simultaneously, the Navy contacted Nakajima and instructed them to be ready to develop a long-range bomber from the DC-4E when it arrived in Japan.

Shortly after the DC-4E was brought to Japan in 1939, Nakajima secretly took delivery of the plane and dismantled it for close study. Using the American aircraft’s wing, undercarriage, and powerplant installation as starting points, Nakajima created for their bomber design a new fuselage with a glazed nose, a rather pregnant-looking bomb-bay that seemed to drag the ground, a “stepped" ventral gun position facing aft just behind the bomb-bay, and new tail surfaces with twin fins and rudders. The DC-4E had had a third central fin for stability, similar to that of the postwar Lockheed Constellation, but the bomber design required a clear field of fire for the dorsal turret’s 20mm cannon. The final effect was to create an aircraft roughly similar to the initial configuration of the Consolidated B-24 Liberator. Powered by four 1,870-hp Nakajima Mamoru 11 fourteen-cylinder engines driving four-bladed propellers, the first Experimental 13-Shi Attack Bomber G5N1 Shinzan (as the plane was officially designated) first flew on April 10, 1941.

Unfortunately for the Japanese, the Shinzan was based on an unsuccessful airliner rejected by every American airline, and their inexperience in designing such large aircraft led to the G5N1 being plagued by over-complexity, excessive weight, and unreliable engines. The Shinzan was slow, lacked range, and had poor altitude performance by international standards for four-engined bomber aircraft. Although Nakajima built three more G5N1 prototypes and attempted to rescue the design by fitting four 1,530-hp Mitsubishi Kasei 12 engines in two further prototypes designated G5N2, the Navy canceled the program in 1942. Eventually, two of the G5N1s were re-engined with Kasei 12s and were operated alongside the two G5N2s as freight transports, all four planes being given the official designation G5N2-L Shinzan-Kai Model 12 Transport, and identified by the Allies by the code name Liz. These planes toiled in obscurity for the remainder of the war; at least one survived to be photographed more or less intact at Atsugi Naval Airfield in August 1945, looking very tired and ready for pasture – or the scrap heap.

Further DC-4E-based variants, projected but not proceeded with, included the Ki-68, designed by Nakajima for the Japanese Army and powered by either four Mitsubishi Ha-101 or four Nakajima Ha-103 engines, and the Ki-85, designed by Kawanishi and intended to be powered by four Mitsubishi Ha-111Ms.

Had the G5N entered combat service, one suspects it would’ve been plagued by most of the same problems Germany’s Focke-Wulf Fw 200 Condor experienced in its lifetime – but without the opportunity to become the “Scourge of the Pacific" as its German counterpart had been the “Scourge of the Atlantic," if only briefly.