The D4Y-I was the world’s first four-engine bomber. It had a range of 2,100 miles and could carry 1,000lbs of bombs or six torpedoes at 500mph but never saw combat in World War II due to its high cost and limited production. If not for this strange history of warplane neglect, it might have made an impact on the battlefronts like other air forces did with their own bombers during that time period.

The “four engine japanese bomber” is a type of aircraft that never saw combat. This is because the Japanese army was focused on fighting in China and Southeast Asia.

Why Japan's Forgotten Four-Engine Bomber Never Saw Combat

The four-engine Nakajima G8N bomber came too late in World War II, when Japan had already switched to defensive mode.

The four-engine, long-range strategic bomber is notable by its near absence from Japan’s combat arsenal, given the enormous expanse of the Pacific Theater of Operations during World War II. Furthermore, although the Japanese Army Air Force was satisfied with twin-engine medium bombers, the navy was fixated on developing long-range multiengine attack aircraft. This was because Japanese naval policy was based on the assumption that as the bigger US Navy made its way across the Pacific, it would be methodically weakened by a series of torpedo assaults by Japanese submarines, destroyers, and long-range strike aircraft.

The Mitsubishi G3M and G4M were initially effective in this capacity, notably sinking the British battleship Prince of Wales and battlecruiser Repulse off the coast of Malaya on December 10, 1941. Despite this, the Imperial Japanese Navy (IJN) wanted an aircraft that was even bigger, better equipped, and had a greater range. Despite the fact that the Japanese aviation industry had no expertise building four-engine aircraft, a chance to acquire a state-of-the-art foreign-built model for study arose in 1939. 

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

The Douglas DC-4E was first flown in 1938 as a prototype for a four-engine, pressurized transcontinental airliner. It was rejected by US carriers because it was too heavy, underpowered, and costly. As a result, Douglas Aircraft welcomed the opportunity to recover some of the DC-4E’s development expenses by selling the prototype to the Japanese for commercial usage. However, once in Japan, the plane was handed over to experts at the Nakajima Aircraft Company, who studied the design and structural features in order to develop a long-range naval assault bomber.

The G5N Shinzan was born as a consequence (Deep Mountain). The G5N was 101 feet 9 inches long and had a wingspan of 138 feet 2 inches when it first flew on April 8, 1941. It had a maximum takeoff weight of 70,500 pounds and weighed 44,000 pounds empty. When Allied air intelligence discovered of the jet’s existence, it was given the nickname Liz, which sounds like a tiny name for such a massive plane. The wings and tricycle landing gear of the G5N were similar to those of the DC-4E, but the fuselage and tail were completely different. The Shinzan, like the DC-4E, inherited the DC-4E’s flaws of being overweight and underpowered, along with its wings and landing gear. The bomber’s four 14-cylinder Nakajima Mamori engines, each rated at 1,870 horsepower, proved insufficient and unreliable in the end. Only six prototype Shinzans were ever constructed as a consequence. The United States, for example. The G5Ns, the Army Air Corps’ enormous Boeing XB-15 bomber prototype, were assigned to long-range transport duties.

In 1943, the IJN published a new request for a long-range, four-engine attack aircraft, undeterred by the G5N’s failure. Japan had acquired access to more advanced US bomber technology by that time. In 1942, at least three Boeing B-17 Flying Fortresses were seized in the Philippines and the Dutch East Indies, then rebuilt and tested in Japan. The revised specification asked for a 370 mph top speed, an 8,000-pound bombload, and a range of 4,600 miles. 

Americans examine a G8N on an airfield at Koizumi. (San Diego Air and Space Museum) On an airstrip near Koizumi, Americans inspect a G8N. (Photo courtesy of the San Diego Air and Space Museum)

After just 18 months of development, Nakajima was able to fly its new bomber for the first time on October 23, 1944. The aircraft was nicknamed Rita by the Allies and was designated as the G8N Renzan (Mountain Range). The G8N was 75 feet 3 inches long and 106 feet 9 inches wide, much smaller than its predecessor and 6,000 pounds lighter at 38,000 pounds empty. The G8N, on the other hand, had a maximum takeoff weight of almost 70,900 pounds when fully loaded, which was somewhat more than the fully laden G5N. The G8N was powered by four turbosupercharged 2,000-hp Nakajima Homare 18-cylinder radials, which provided approximately 25% more power than the Mamoris on the G5N, allowing it to fly at considerably higher speeds and service ceilings. The Renzan also had a range of 4,500 miles, almost 2,000 miles longer than the G5N. 

The G8N’s defensive armament was another area where it outperformed the previous G5N. Two 20mm cannons and four 7.7mm machine guns were placed aboard the Shinzan, with only one of the cannons in a power-driven turret. The Renzan, on the other hand, was equipped with six 20mm cannons in powered dorsal, ventral, and tail turrets, as well as two 13mm machine guns in a powered nose turret and two 13mm guns in flexible waist mounts. Although the mid-wing G8N was in no way a duplicate of the low-wing American bomber, it was certainly no accident that its defensive configuration was similar of the B-17’s.

The Renzan was a powerful aircraft when compared to similar Allied bombers in service at the time. However, by late 1944, the war’s trajectory had shifted dramatically. The IJN no longer needed big long-range marine attack aircraft or strategic bombers since Japan was on the defensive. Furthermore, the light alloys needed for the production of such aircraft were becoming rare, and the remaining resources were mostly used to produce defensive fighters. Bombing attacks on Japanese industry by Boeing B-29s from the Mariana Islands hindered the production of the G8N even further. 

A Renzan with propellers removed sits at Nakajima’s Koizumi plant after the war. (National Archives) After the war, a Renzan with the propellers removed rests at Nakajima’s Koizumi factory. (From the National Archives)

As a consequence of all of these difficulties, just four Renzan bombers were built before the war’s conclusion, and none of them were ever deployed in combat. One G8N was destroyed on the ground during an Allied air assault, but one of the surviving prototypes was sent to the United States after the war for examination. Although the Army Air Forces identified a few flaws, these were minor issues that might have arisen on any prototype and could have been worked out over time. The G8N seems to have impressed the AAF on the whole. Unfortunately for posterity, when the AAF completed its testing of the G8N, this once-in-a-lifetime Japanese warplane was abruptly retired.  

The original version of this article was published in the September 2021 edition of Aviation History. Make sure you don’t miss an issue by subscribing!

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The “super hornet” is a four-engine, twin-tailed bomber that was designed to replace the older American B-17 Flying Fortress and B-24 Liberator bombers. The design of the plane was largely based on the German Arado Ar 234 jet bomber. However, it never saw combat during World War II because Japan surrendered before they could be used in battle.

Frequently Asked Questions

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