It was built by the Kawasaki Locomotive Works in 1936 as a mixed-traffic engine, and was one of the most successful and most numerous types of steam locomotives produced in Japan. (Even during World War 2, they were used to deliver weapons and vehicles across Japan to supply the Japanese Imperial Army troops.)
The Class D51 inspired the design of many other types of steam locomotives, was one of the first to use a knuckle-style coupler, and eventually became an iconic steam locomotive in Japan. (Like with the American Standard in the US.)
Many were built, and many are still preserved, as well as some operate steam excursions.