The Kyushu K9W Momiji was a licence-built version of the Bücker Bü 131B Jungman basic trainer produced in Japan for use by the Imperial Japanese Navy. In 1938 the Bücker company demonstrated one Bü 131 Jungmann and one Bü 133 Jungmeister in Japan. The Japanese navy was impressed by the aircraft, and purchased the Jungmann. This aircraft was subjected to strenuous trials, in which it continued to impress. Twenty more Jungmann were ordered in 1939 as the Navy Experimental Type Bu Primary Trainer, and were tested as training aircraft.
The Japanese navy was impressed with the performance of the Jungmann, but in 1939 didn't want to become reliant on foreign designs. Hitachi and Watanabe were both ordered to produce a trainer similar to the Jungmann, but different enough not to count as a copy. Both companies produced aircraft in 1941, in Watanabe's case one monoplane and one biplane, but none of the competitors could match the German original, and so a Japanese representative was sent to Germany to purchase a production licence.
The Jungmann entered production in Japan in August 1942 as the Watanabe K9W1 Navy Type 2 Primary Trainer Model 11. The name was changed after Watanabe was renamed as Kyushu, although the company code letter remained the same, and the aircraft is normally referred to as the Kyushu K9W1.
The K9W1 was a standard biplane, with a metal framework and fabric and wood covering. It was powered by the 110hp Hitachi GK4A Hatsukaze 11 air-cooled radial engine, and was produced by Kyushu and Hitachi. Sources disagree on the number of aircraft produced, although a total of 339 aircraft seems most likely.
The performance of the K9W1 interested the Japanese Army, but instead of adopting the Navy's trainer, it placed the aircraft in production with a different manufacturer, as the Kokusai Ki-86.