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Nakajima G5N Shinzan (Deep Mountain)

Four-Engine Heavy Bomber Prototype

Imperial Japan | 1941

"To expedite development, Nakajima used the existing, abandoned American Douglas DC-4E airliner to form the basis of their IJN four-engined heavy bomber proposal."

Authored By: Staff Writer | Last Edited: 06/01/2020 | Content ©www.MilitaryFactory.com | The following text is exclusive to this site; No A.I. was used in the generation of this content.
Much like the Germans during World War 2 (1939-1945), the Empire of Japan never truly found a notable four-engined "heavy", a long-ranging multi-engined strategic bomber capable of dropping considerable war loads on enemy targets over distance. Projects were had by both parties but none materialized beyond paper drawings or prototypes. In the early-war period, the Nakajima Aircraft Company attempted such a project based around the proven framework of the American Douglas DC-4E that it had acquired - but even this initiative fell to naught as just six of the type were produced in all.

The DC-4E was, itself, a developmental model arranged by the Douglas Aircraft Company and eventually operated by American Airlines in the United States and, later, Imperial Japanese Airways. The original aircraft was powered by 4 x Pratt & Whitney R-2180-S1A1-G "Twin Hornet" air-cooled radial piston engines driving three-bladed propeller units and outputting 1,450 horsepower each and one notable physical characteristic of the design was the three-finned tail unit. The aircraft was crewed by three and could carry as many as 42 passengers over distances of 2,200 miles. A first-flight was recorded on June 9th, 1939 - this before World War 2 officially began.

After some testing, Douglas decided against the DC-4E and this led Japan to procure the abandoned prototype while Douglas pursued another in-house design that became the classic "DC-4" aircraft of the World War 2 period. As such, the DC-4E initially fell to Imperial Japanese Airways in the latter part of 1939 and was seen by the Imperial Japanese Navy (IJN) as a possible candidate for conversion into a long-range bomber. The Japanese reverse-engineered the American airliner to become the short-lived "G5N" under the Nakajima Aircraft Company brand label. To cover the work, Japanese media reported the DC-4E as crashed while the true aircraft could be reviewed under cover and modified by Japanese industry to produce the proposed bomber form. As the DC-4E was readily available for the conversion work, it was selected as no local solution could be had from Japanese aero-industry at the time.

The G5N was drawn up to fulfill an IJN requirement for a long-range strategic bombing platform capable of crossing the vast distances of the Pacific. The service, like the Imperial Japanese Army (IJA), lacked such a performer while other powers of the world had already embraced the concept of a strategic bomber force built on range and firepower. For the IJN, the minimum acceptable operational range became 3,500 miles, giving what would become their next bomber excellent "legs" in the air.

In common with contemporary bombers of the day, the G5N was completed with metal skinning (save for the control surfaces) and had its triple-rudder tail unit simplified to a more modern twin-fin arrangement. For power there was 4 x Nakajima NK7A "Mamori" 11 series air-cooled radial piston engines of 1,870 horsepower each, these driving either three-or-four-bladed propeller units. Internally, the requisite bomb bay was carved out of the passenger section and the nose was glazed over for better viewing by the bombing crew. The flight deck was positioned aft and above the bombardier's compartment and was equally heavily glazed for viewing by the flight crew. Ground-running would be accomplished by the DC-4E's original tricycle landing gear arrangement involving main legs under each inboard engine nacelle and a twin-wheeled nose leg under the forward mass of the fuselage.

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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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Power & Performance
Those special qualities that separate one aircraft design from another. Performance specifications presented assume optimal operating conditions for the Nakajima G5N Four-Engine Heavy Bomber Prototype.
4 x Nakajima NK7A "Mamori 11" 14-cylinder air-cooled radial piston engines developing 1,871 horsepower each driving four-bladed propeller units.
261 mph
420 kph | 227 kts
Max Speed
24,442 ft
7,450 m | 5 miles
Service Ceiling
2,647 miles
4,260 km | 2,300 nm
Operational Range
City-to-City Ranges
Operational range when compared to distances between major cities (in KM).
The nose-to-tail, wingtip-to-wingtip physical qualities of the Nakajima G5N Four-Engine Heavy Bomber Prototype.
101.8 ft
31.02 m
O/A Length
138.2 ft
(42.12 m)
O/A Width
28.9 ft
(8.80 m)
O/A Height
44,313 lb
(20,100 kg)
Empty Weight
70,548 lb
(32,000 kg)
Design Balance
The three qualities reflected below are altitude, speed, and range. The more full the box, the more balanced the design.
Available supported armament and special-mission equipment featured in the design of the Nakajima G5N Shinzan (Deep Mountain) Four-Engine Heavy Bomber Prototype .
1 x 20mm Type 99 Model 1 automatic cannon in powered dorsal turret at the nose.
1 x 7.7mm Type 92 machine gun on trainable mounting in nose position.
1 x 7.7mm Type 92 machine gun on trainable mounting in ventral, rear-facing position.
1 x 7.7mm Type 92 machine gun on trainable mounting at port side waist position.
1 x 7.7mm Type 92 machine gun on trainable mounting at starboard side waist position.
1 x 20mm Type 99 Model 1 automatic cannon in powered turret at tail position.

Internal bomb load of unknown rating consisting of conventional drop bombs and possibly torpedoes.
Notable series variants as part of the Nakajima G5N Shinzan (Deep Mountain) family line.
G5N "Shinzan" ("Deep Mountain") - Base Series Designation.
"Experimental 13-Shi Attack Bomber" - Imperial Japanese Navy (IJN) designation.
G5N1 - Initial prototypes; two completed to the standard.
G5N2 - Secondary prototypes; four completed to the standard with 4 x Mitsubishi MK4B 12 "Kaisei" radial engines driving four-bladed propeller units.
G5N2-L "Shinzen-Kai" - Reconstituted G5N2 bombers for the long-range transport role by the IJN.
Ki-68 - Proposed IJA model with either Mitsubishi or Nakajima engines.
Ki-85 - Proposed IJA model with Mitsubishi Ha-111M engines; mock-up completed in 1942 but design cancelled i May of 1943.
"Liz" - Allied Codename.
Global customers who have evaluated and/or operated the Nakajima G5N Shinzan (Deep Mountain). Nations are displayed by flag, each linked to their respective national aircraft listing.

Total Production: 6 Units

Contractor(s): Nakajima Aircraft Company - Japanese Empire
National flag of modern Japan

[ Imperial Japan (cancelled) ]
Relative Max Speed
Hi: 300mph
Lo: 150mph
Aircraft Max Listed Speed (261mph).

Graph Average of 225 MPH.
Era Crossover
Pie graph section
Showcasing Aircraft Era Crossover (if any)
Max Alt Visualization
Small airplane graphic
Production Comparison
Entry compared against Ilyushin IL-2 (military) and Cessna 172 (civilian) total production.
MACH Regime (Sonic)
RANGES (MPH) Subsonic: <614mph | Transonic: 614-921 | Supersonic: 921-3836 | Hypersonic: 3836-7673 | Hi-Hypersonic: 7673-19180 | Reentry: >19030
Aviation Timeline
1 / 1
Image of the Nakajima G5N Shinzan (Deep Mountain)
Image from the Public Domain.

Mission Roles
Some designs are single-minded in their approach while others offer a more versatile solution to airborne requirements.
Some designs stand the test of time while others are doomed to never advance beyond the drawing board; let history be their judge.
Going Further...
The Nakajima G5N Shinzan (Deep Mountain) Four-Engine Heavy Bomber Prototype appears in the following collections:
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