Key Points and Summary – Japan’s Design A-150 “Super Yamato” was meant to be the ultimate battleship: up to 70,000–90,000 tons, six 510 mm guns, huge armor belts, and 30-knot speed to crush any U.S. rival.
-Conceived after Japan walked away from naval treaties, it embodied Tokyo’s bet on quality over quantity in a future decisive fleet battle.
Yamato-Class Battleship from WWII. Image Credit: Creative Commons.
-But industrial limits, cost, steel and gun challenges, and the rapid rise of carrier airpower doomed the project before a keel was ever laid.
-In hindsight, A-150 looks less like a missed opportunity and more like a giant that would have been doomed from above.
Inside Japan’s Cancelled A-150 Super Yamato Battleship Project
In the immediate aftermath of WWI, some of the great powers of the time, the U.S., the UK, Italy, France, and Japan, signed the London Naval Treaty in 1930, which imposed size and tonnage limits.
This effectively prohibited the creation of large battleships and heavily regulated submarine warfare, seeking to avoid battles seen in World War I.
This treaty was later discarded with the onset of WWII. This fact benefited the navies of Japan and the U.S. Throughout the 30s, the Imperial Japanese Navy (IJN) experimented with various types of battleships.
One design that never left the drawing board was Design A-150, which was an experimental super-battleship. Although it had a fearsome design, it was ultimately never laid down due to material and economic concerns.
Development of Japan’s “Super Battleship”
In the late 1930s, the IJN operated under a doctrine that emphasized qualitative superiority over numerical parity. Japanese strategists understood that they could never match the United States in sheer industrial output, so they aimed to build ships so powerful that each could defeat multiple enemy vessels.
This philosophy had already produced the Nagato-class and later the Yamato-class battleships, which carried the largest naval guns of the time.
Yet even before the Yamato-class was completed, Japanese planners anticipated that the U.S. would respond with larger and more powerful ships.
Montana-Class Battleship. Image Credit: Creative Commons.
To maintain superiority, Japan began planning an even more formidable class: Design A-150, which is unofficially referred to as the Super Yamato class.
These ships were intended to be the ultimate expression of big-gun naval warfare, ensuring Japan’s dominance in a decisive fleet engagement.
The A-150 was conceived around 1938–1939, with design work completed mainly by early 1941.
Although exact details were destroyed at the end of the war, surviving fragments and expert reconstructions allow historians to outline its intended characteristics.
The ship was projected to displace roughly 70,000 tons, though early drafts envisioned up to 90,000 tons, making it the largest battleship ever conceived.
Its length would have been approximately 263 meters, with a beam of nearly 39 meters. The designers aimed for a speed of about 30 knots, faster than contemporary U.S. battleships like the North Carolina class.
Design A-150
Most striking of all was its main armament: six 510 mm (20.1-inch) guns mounted in three twin turrets. These would have been the largest naval guns ever built, surpassing even Yamato’s 460 mm weapons.
Secondary armament would have included numerous 100 mm dual-purpose guns for anti-aircraft and surface defense. Its armor protection was equally ambitious, with a side belt estimated at 460 mm thick and turret faces up to 800 mm.
Top of USS Iowa. Image Credit: National Security Journal.
However, Japanese steel mills struggled to produce single plates of this size, forcing consideration of double-layered armor, which was less effective. In short, the A-150 was envisioned as a floating fortress capable of annihilating any opponent in a gun duel.
Japan’s withdrawal from the London Naval Treaty in 1936 freed it from tonnage restrictions, enabling such ambitious projects. The IJN feared that the United States, with its vast industrial base, would eventually outbuild Japan three-to-one in capital ships.
To counter this, Japan embraced quality over quantity.
The A-150 was intended as a decisive battle weapon, capable of tipping the scales in a single engagement. Yet this vision was rooted in assumptions that were already becoming outdated. Despite some awareness of carrier potential, many admirals still believed that battleships would decide wars, as they had in previous decades.
Why A-150 Was Never Built
Despite its grand vision, the A-150 never left the drawing board.
There were multiple reasons for this. First were industrial and material limitations. Japan’s steel industry could not produce the massive armor plates required for the A-150, and the proposed solution of double-layered plates was inferior and complicated. Manufacturing 510 mm guns posed enormous technical challenges.
While Japan had successfully built 480 mm guns in the 1920s, scaling up to 510 mm was a leap that required time and resources Japan did not have.
Economic constraints compounded these difficulties. The projected displacement of 90,000 tons made the A-150 prohibitively expensive. Even the Yamato-class strained Japan’s finances and shipyard capacity, and wartime demands for oil, aircraft, and merchant shipping diverted resources away from super battleships.
As WWII amped up, it was clear that naval warfare was changing. By 1941, carrier aviation had demonstrated its potential, notably in the British attack on Taranto. Japanese planners began prioritizing carriers for the upcoming Pacific War.
Long View of USS Iowa Guns. Image Credit: National Security Journal.
The attack on Pearl Harbor and subsequent carrier battles at Coral Sea and Midway confirmed that air power, not battleships, would define contemporary naval warfare.
The labor, industrial, and infrastructural needs that the project required were too much for Japan, especially during a full-scale war. Propulsion systems capable of driving a 90,000-ton ship at 30 knots were beyond Japan’s practical engineering capacity at the time.
The leadership in the IJN refused to accept the changing trends in naval warfare. It clung to the decisive battle doctrine even as aircraft carriers rendered it obsolete. While there were other voices in the IJN, the traditional battle doctrine remained supreme.
Ironically, Yamato and Musashi themselves were sunk by an air attack launched from American carriers.
Had the A-150 been built, it likely would have met a similar fate and would have been remembered as a monumental waste of everyone’s time and resources.
About the Author: Isaac Seitz
Isaac Seitz, a Defense Columnist, graduated from Patrick Henry College’s Strategic Intelligence and National Security program. He has also studied Russian at Middlebury Language Schools and has worked as an intelligence Analyst in the private sector.
