The Nakajima G5N Shinzen (Mountain Recess) was a four-engined heavy bomber that was based on the Douglas DC-4E transport aircraft.
The DC-4E had been developed in the late 1930s. In 1936 funding had been raised to build a single prototype of this large 42-seat airliner. The prototype was completed by May 1938. It had a fuselage similar to the DC-2, but greatly enlarged. The wings had straight but tapered leading and trailing edges and the tail had three vertical surfaces - one central and one on each end of the horizontal tail surfaces. The prototype was tested by United Air Lines, and disappointed. As a result work on the four-engined DC-4 was cancelled, work on the more familiar DC-4/ C-54 was begun and the four engined prototype became the DC-4E.
Late in 1939 the DC-4E was purchased by the Mitsui Trading Company, officially for use by Dai Nippon Koku (Greater Japan Air Lines). Douglas shipped the aircraft across the Pacific, re-assembled it in Japan and handed it over. After a few flights the Japanese reported that the aircraft had crashed.
In fact it had been handed over to Nakajima, who had been given the task of constructing a heavy bomber with a range of 3,450 to 4,030 miles, to meet a 13-Shi specification. Nakajima's engineers worked very quickly. The DC-4E was dismantled and a new design was quickly produced. This kept the engine installation (but with Nakajima NK7A Mamoru engines), wings and undercarriage of the DC-4E, but with a new fuselage that included a ventral bomb-bay and a twin fin and rudder tail. The prototype G5N1 flew in December 1939. It was the first four-engined land-plane to fly for the Japanese Navy and the first aircraft built in Japan to use a retractable tricycle landing gear.
The G5N was not a successful design. The DC-4E had been rejected by the American airlines because it was over-complex and uneconomic. The changes needed to turn it into a military aircraft increased its weight, and the Mamoru engines proved to be unreliable. As a result the G5N suffered from poor performance, with a top speed of only 261mph and a low operating altitude of 13,450ft. Three more G5N1 prototypes were completed and they were followed by two G5N2 prototypes powered by 1,530 Mitsubishi Kasei 12 engines. Although the Kasai 12 was more reliable it was still underpowered and the entire project was abandoned.
Two of the G5N1s were given Kasei 12 engines. They and the two G5N2s were converted into freight transports, as the Shinzan-Kai Model 12 Transport G5N2-L. Although there were only four of these aircraft they did attract Allied attention and got the code name 'Liz'.
Two variants of the G5N had been planned for the Japanese Army, the Nakajima Ki-68 and the Kawanishi Ki-85. Both of these designs were abandoned.
Engine: Four Nakajima Mamoru radial engines
Power: 1,870hp each
Crew: 7 to 10
Span: 138ft 3 1/16in
Length: 101ft 9 1/4in
Empty weight: 44,313lb
Maximum take-off weight: 70,778lb
Max speed: 261mph at 13,450ft
Cruising speed: 230mph at 13,125ft
Service ceiling: 24,440ft
Endurance: 2,657 miles with lower bomb load
Armament: Two 20mm cannon, four 7.7mm machine guns
Bomb load: 8,818lb over short ranges, 4,409lb normal