The Nakajima G8N four-engine bomber arrived too late in World War II, when Japan had already gone over to the defensive.

In view of the vast area of the Pacific Theater of Operations during World War II, it is somewhat ironic that the four-engine, long-range strategic bomber is conspicuous by its near absence from Japan’s operational inventory. Moreover, while the Japa­nese Army Air Force was content with twin-engine medium bombers, it was the navy that was obsessed with the development of long-range multiengine strike aircraft. That was because Japa­nese naval strategy was predicated upon the premise that as the larger U.S. Navy made its way across the Pacific, its strength would be systematically whittled down by a series of torpedo attacks by Japanese submarines, destroyers and long-range strike aircraft.

The Mitsubishi G3M and G4M proved initially successful in that role, most dramatically when they sank the British battleship Prince of Wales and battlecruiser Repulse off Malaya on December 10, 1941. Nevertheless, the Imperial Japanese Navy (IJN) desired an even larger, better-armed and longer-ranged airplane. Although the Japanese aircraft industry had no experience designing four-engine aircraft, in 1939 it was presented with an opportunity to purchase a state-of-the-art foreign-built example for analysis. 

Japan’s first attempt at a four-engine bomber was the overweight and underpowered G5N Shinzan, which was based on the Douglas DC-4E. (National Archives)
Japan’s first attempt at a four-engine bomber was the overweight and underpowered G5N Shinzan, which was based on the Douglas DC-4E. (National Archives)

First flown in 1938 as the prototype of a four-engine, pressurized transcontinental airliner, the Douglas DC-4E was rejected by U.S. carriers because they considered it too heavy, underpowered and expensive. Consequently, Douglas Aircraft welcomed the chance to recoup part of the DC-4E’s development costs by selling the prototype to the Japanese, ostensibly for commercial use. Once in Japan, however, the aircraft was turned over to engineers at the Nakajima Aircraft Company to study the design and structural details as the basis for a long-range naval attack bomber.

The result was the G5N Shinzan (Deep Mountain). First flown on April 8, 1941, the G5N was 101 feet 9 inches long and had a wingspan of 138 feet 2 inches. It weighed 44,000 pounds empty and had a maximum takeoff weight of 70,500 pounds. Once Allied air intelligence learned of its existence, the airplane was assigned the codename Liz, which seems rather a small name for such a huge aircraft. Although the G5N’s wings and tricycle landing gear resembled those of the DC-4E, its fuselage and tail were entirely different. Along with its wings and landing gear, however, the Shinzan inherited the DC-4E’s undesirable characteristics of being both overweight and underpowered. The bomber’s four 14-cylinder Nakajima Mamori engines, supposedly capable of generating 1,870 hp apiece, ultimately proved inadequate and unreliable. As a result, only six prototype Shinzans were ever built. Like the U.S. Army Air Corps’ mammoth Boeing XB-15 bomber prototype, the G5Ns were relegated to use as long-range transports.

Undeterred by the G5N’s failure, the IJN issued a revised specification for a long-range, four-engine strike aircraft in 1943. By that time Japan had gained access to more modern U.S. bomber technology. At least three Boeing B-17 Flying Fortresses are known to have been captured in the Philippines and the Dutch East Indies in 1942, then repaired and test-flown in Japan. The new specification called for a speed of 370 mph, 8,000-pound bombload and 4,600-mile range. 

Americans examine a G8N on an airfield at Koizumi. (San Diego Air and Space Museum)
Americans examine a G8N on an airfield at Koizumi. (San Diego Air and Space Museum)

Nakajima managed to fly its new bomber for the first time on October 23, 1944, after only about 18 months of development. Called the G8N Renzan (Mountain Range), the aircraft was codenamed Rita by the Allies. The G8N was 75 feet 3 inches long and had a wingspan of 106 feet 9 inches—markedly smaller than its predecessor and, at 38,000 pounds empty, 6,000 pounds lighter. Fully loaded, however, the G8N had a maximum takeoff weight of nearly 70,900 pounds, slightly heavier than the fully loaded G5N. The G8N was powered by four turbosupercharged 2,000-hp Nakajima Homare 18-cylinder radials, which delivered about 25 percent more power than the G5N’s Mamoris, enabling it to fly at a much higher speed and service ceiling. The Renzan also boasted a 4,500-mile range, almost 2,000 miles more than the G5N. 

Another area in which the G8N greatly improved upon the earlier G5N was its defensive armament. The Shinzan mounted two 20mm cannons and four 7.7mm machine guns, with only one of the cannons installed in a power-driven turret. The Renzan, in contrast, had six 20mm cannons in powered dorsal, ventral and tail turrets; two 13mm machine guns in a powered nose turret; and two 13mm guns in flexible waist mounts. It was probably no coincidence that its defensive arrangement was reminiscent of the B-17’s, although the mid-wing G8N was in no way a copy of the low-wing American bomber.

When judged against comparable Allied bombers in service at that time, the Renzan was indeed a formidable aircraft. By late 1944, however, the course of the war had radically changed. Japan was on the defensive and the IJN no longer required large long-range maritime strike aircraft or strategic bombers. Moreover, the light alloys required for the manufacture of such aircraft were becoming scarce, and what resources remained were chiefly devoted to the manufacture of defensive fighters. Production of the G8N was further hampered by bombing raids on Japanese industry carried out by Boeing B-29s from the Mariana Islands. 

A Renzan with propellers removed sits at Nakajima’s Koizumi plant after the war. (National Archives)
A Renzan with propellers removed sits at Nakajima’s Koizumi plant after the war. (National Archives)

The result of all of those factors was that only four Renzan bombers were completed by the end of the war and none were ever used operationally. One G8N was destroyed on the ground during an Allied air attack, but one of the surviving prototypes was taken to the United States for evaluation after the war. Although the Army Air Forces found fault with a number of details, they were minor matters that might have cropped up on any prototype and could have been ironed out over time. On the whole, the AAF seems to have been impressed with the G8N. Unfortunately for posterity, after the AAF finished testing the G8N, this rara avis among Japanese warplanes was unceremoniously scrapped.  

This article originally appeared in the September 2021 issue of Aviation History. Don’t miss an issue, subscribe!