The D51 was a class of Mikado locomotives built for the rising freight volumes in the thirties. It was a more powerful successor of the D50 and was designed by Hideo Shima. It was lighter than the D50 due to the extensive use of electric welding and was a bit shorter to fit onto 18-metre turntables which were used on branch lines. Another modern feature were boxpok driving wheels. 

The first 95 locomotives had a long casing on top of the boiler that enclosed the domes and the feedwater heater. These were called “Namekuji”, what stood for “Slug”. Later locomotives had a boiler pressure of 15 instead of 14 bars and no casing. The feedwater heater was now located crosswise in front of the chimney. In World War II, construction methods and materials were simplified. Now some parts were made of wood, like the smoke deflectors. These locomotives suffered from reliability issues and even boiler explosions.

In total, 1115 locomotives were built for the JNR, making it their most numerous steam locomotive. 37 more were built for Taiwan, ten for the Philippines and two for Korea. After World War II, 30 were left on Sakhalin and used by the SŽD as Д51 until 1979. The country with the shortest career were the Philippines, where the Manila Railroad designated them class 300. They had been introduced in 1951 and were replaced by diesel locomotives in 1956.

In Japan, six were rebuilt into the 2-8-4 class D61 in 1960 and 1961. The direct successor of the D51 was the D52 that also had the wheel arrangement 2-8-4. The last D51 was withdrawn in December 1975, only some months before the general end of steam in Japan. A total of 180 have been preserved, 174 of these in their country of origin. Five are still able to run under their own power, but three of these have been converted to run on compressed air.