On April 8, 1941, at 2:35 in the afternoon, a massive four-engine aircraft rolled down the runway at Nakajima’s test facility outside Tokyo. Over one hundred feet long with a wingspan approaching one hundred forty feet, it was the largest land-based aircraft Japan had ever built. Engineers from the Imperial Japanese Navy watched as the bomber struggled into the humid air. It climbed slowly, the controls sluggish, the engines straining.

They had spent eighteen months building it. They had spent $950,000 buying the American airliner it was based on, and they had faked a crash to hide what they were really doing. The aircraft was Japan’s first four-engine strategic bomber, and it was already failing. The story began in 1935, during the golden age of American commercial aviation.

William Patterson, president of United Airlines, approached the Douglas Aircraft Company with an audacious vision. He wanted a four-engine airliner capable of crossing the continent without stopping, carrying forty-two passengers by day or thirty in sleeper berths at night. It would be pressurized, air-conditioned, and represent the cutting edge of aviation technology. Douglas agreed to proceed when several other airlines joined the effort.

American, Eastern, Pan-American, and Transcontinental and Western Air each contributed $100,000 toward development. The engineering challenge was unprecedented. Arthur Raymond, who had led the DC-3 program, took charge. The team produced 8,100 drawings and conducted 866 wind tunnel tests totaling 1,100 hours.

The finished aircraft, called the DC-4E, was a marvel. It was the first large aircraft with tricycle landing gear, allowing it to sit level on the ground. Its triple tail was designed so the aircraft could fit inside existing airport hangars. Four Pratt and Whitney R-2180 Twin Hornet engines, each producing 1,450 horsepower, powered the design.

The interior offered a level of luxury passengers had never seen. On June 7, 1938, chief test pilot Carl Cover lifted the DC-4E into the California sky for its maiden flight from Clover Field in Santa Monica. Testing continued through the summer, and on June 9, 1939, Orville Wright himself came aboard for a flight over Dayton, Ohio, connecting the inventor of powered flight to the most advanced airliner in the world. But the triumph did not last.

United’s in-service evaluation revealed serious problems. The aircraft was too complex for practical airline operation. Its systems baffled mechanics, it consumed fuel at alarming rates, and its range fell short of predictions. As engineers had added systems and strengthened structures, the aircraft’s weight had grown from a projected 50,000 pounds to 65,000 pounds.

Performance suffered across the board. One by one, the sponsoring airlines withdrew. Pan-American and TWA had already left before the first flight. United, American, and Eastern reached the same conclusion: the DC-4E was impressive technology but unsuitable for commercial service.

Douglas faced an expensive embarrassment. They had invested roughly $3 million of their own money in an aircraft no one wanted. The consortium paid $91,250 each to settle their commitments. Douglas decided to design a new, simpler four-engine airliner based on the lessons learned, and quietly tried to sell the prototype to recover some costs.

Late in 1939, representatives from Japan’s Mitsui Trading Company approached Douglas. They wanted to purchase the DC-4E for Japan Airlines. The airline was expanding as Japan’s empire grew, and the aircraft represented technology far beyond anything available in Japan. Douglas saw a legitimate commercial sale and accepted the $950,000 offer.

They did not know that Japan Airlines was a front for the Imperial Japanese Navy. The Navy had been watching American aviation developments closely. Japan had achieved success with single-engine fighters like the Zero and twin-engine bombers like the G4M Betty, but they had never built a four-engine aircraft. A strategic bomber capable of crossing the Pacific required four engines, and Japanese engineers did not yet know how to build one.

The DC-4E seemed like the answer. They would buy it, study it, and use its technology to build their own bomber. On September 29, 1939, workers at Douglas’s Long Beach factory disassembled the DC-4E and loaded it onto a freighter bound for Japan. Along with the aircraft went the complete engineering data: drawings, test results, performance figures, maintenance manuals, even the wind tunnel model.

Douglas technicians traveled to Japan to help reassemble the aircraft, unaware they were teaching potential enemies how to build strategic bombers. After reassembly and brief test flights, the DC-4E was quietly transferred to Nakajima Aircraft Company. The Japanese government fabricated a cover story: the airliner had crashed into Tokyo Bay shortly after takeoff from Haneda. Nothing was recovered.

The matter was closed. In reality, the aircraft sat in a Nakajima hangar being methodically dismantled, with every component measured, photographed, and documented. What emerged was the Nakajima G5N, designated the Experimental Type 13 Attack Bomber and given the name Shinzan, meaning Deep Mountain. The wing, tricycle landing gear, and overall configuration were copied directly from the DC-4E.

The fuselage was redesigned for military use, with a bomb bay capable of carrying up to 4,000 kilograms of bombs or two torpedoes. Defensive armament included two 20 mm cannon and four 7. 7 mm machine guns. The triple tail was replaced with a twin fin arrangement.

For power, Nakajima chose its own ambitious NK7A Mamoru engines, a 14-cylinder radial supposedly producing 1,870 horsepower. The first prototype was completed in early 1941. On April 8, 1941, it made its maiden flight. The test pilots’ reports were damning.

The Mamoru engines did not deliver their promised power. They ran rough, overheated during climbs, and failed frequently. The flight controls felt sluggish. Maximum speed was only 261 mph, slower than contemporary American and British heavy bombers.

The service ceiling was under 25,000 feet. Most critically, the range fell to about 2,600 nautical miles, short of the 3,000-mile minimum the Navy had specified. The dream of bombing Pearl Harbor or the American mainland was clearly impossible. Nakajima built additional prototypes, hoping refinements would solve the problems.

Engineers modified cowlings, adjusted propeller pitch, and tweaked the fuel system. Performance improved only slightly. The engines continued to fail, forcing engineers to detune them for reliability, which further reduced their inadequate power. In a desperate attempt to salvage the program, Nakajima built two prototypes with proven Mitsubishi Kasei engines.

The Kasei produced only 1,530 horsepower, and performance became truly dismal. The G5N2 flew even slower and climbed even more poorly. By 1942, the verdict was clear. The Shinzan was a failure.

The aircraft was simply too heavy for any available Japanese engine. The Imperial Japanese Army had briefly considered its own variants, the Ki-68 and Ki-85, and even built a full-scale mockup. But watching the Navy’s struggles, the Army cancelled its program in May 1943 without producing a single flying aircraft. The Navy refused to waste the aircraft entirely.

Four of the six prototypes were converted into long-range transports, designated G5N2-L and renamed Shinzan Kai. The bomb bays were sealed, the armament removed, and the crew reduced to six. Assigned to the 1021st Naval Air Squadron, they flew supply missions across the Japanese Empire, hauling cargo and personnel to bases in Hong Kong, Formosa, the Philippines, Tinian, and the Marianas. The bomber intended to strike terror into American hearts spent the war as a freight hauler.

American intelligence eventually detected the G5N through aerial reconnaissance and assigned it the code name Liz, expecting it to be a dangerous strategic bomber. American commanders prepared contingency plans for attacks that never came. They did not realize Japan had abandoned the aircraft as a combat platform years earlier. Carl Cover did not live to see the war’s end.

After Pearl Harbor, he returned to military service as a colonel overseeing B-29 production at Bell Aircraft in Georgia. On November 27, 1944, he died in a plane crash during a snowstorm near Dayton, Ohio. None of the six G5N aircraft survived the war. Some were destroyed by American air attacks, others cannibalized for spare parts as Japan’s industrial base collapsed.

By the time Allied forces occupied Japan, no complete G5N remained. The DC-4E prototype was stripped down to its components and scraped for materials. The wind tunnel model painted in Japan Airlines colors may have survived, but its location is unknown. The full story only emerged after Japan’s surrender in August 1945, when Allied investigators discovered documents describing the Shinzan program.

They learned about the purchase, the fake crash, the reverse engineering, and the disappointing results. The irony was profound. The United States had rejected the DC-4E because it was too complex and expensive. Japan purchased it hoping to turn American technology against America, but inherited all its flaws and added new ones of their own.

Japanese engineers could see what Douglas had built, but they could not see the reasoning behind it. They did not know which features had been chosen reluctantly as compromises, or which systems had caused endless headaches during development. They lacked the institutional memory that guided Douglas’s later work. When the new, simplified DC-4 emerged in 1942, it became one of the most successful transport aircraft in history.

Over 1,200 were built, serving as the C-54 Sky Master across the Atlantic and Pacific, supporting the Berlin Airlift, and flying for airlines around the world for decades. Japan spent $950,000 to purchase the DC-4E, years reverse engineering it, and built six prototypes. In the end, they had four cargo planes and a lesson in the limits of technological imitation. The Deep Mountain was supposed to cast a shadow across the Pacific.

Instead, it became a cautionary tale about the difference between copying someone else’s work and truly understanding it.