The class 42 or KDL 3 war locomotive was a compromise between the class 52 war locomotive (KDL 1) and the class 44 heavy freight locomotive. It was developed because the class 52 did not always have sufficient power for heavier freight trains, but the axle loads of the bigger sister were too high for many lines. Austria, which was annexed in 1938, and Poland, which was conquered later, were to be the preferred areas of operation.

As a basis, a boiler with the same dimensions as that of the 44 was used and placed on the chassis of the 52. It had only two cylinders, what resulted in very rough running when operating at high speeds and when the entire power was called up. The construction of the boiler of the two pre-production models from 1943 were of the Brotan design developed in Austria. Since this did not provide the desired power, a conventional boiler was used in series production from 1944. Compared to the class 44, the tubes were shortened to save weight.

In service, the typical weaknesses of the war locomotives, which were partly caused by poor quality production, quickly became apparent. Some locomotives had so many defects that they had to be stored damaged after a test run. The closed cab offered protection from the cold in winter, but also led to unbearably high temperatures in summer. In addition, the two injectors were installed inside the cab and generated additional heat.

Production took place at several manufacturers in Germany, Austria and Poland, which were combined in the Gemeinschaft Großdeutscher Lokomotivfabriken (Community of Greater German Locomotive Factories). Due to the course of the war, however, the target of initially 8,000 and later 3,300 locomotives could not be achieved. Only 844 were completed by the end of the war. After the war, many serviceable locomotives of this class were not only left in Germany, but also in Poland and Austria. Some were also scattered in other countries. However, as most of these were in very poor condition, a lot of work had to be done before they could run again.

Simultaneously with the repairs, there were efforts in the post-war period to improve the structural defects of the war locomotives through rebuilds. In addition, there were still unfinished locomotives in many places, from which further locomotives could be built. Thus the total number rose to 1,061 by 1949. Since there were a large number of steam locomotives which did not have the simplifications of the war locomotives in German-speaking countries, the class 42 did not last very long there. The last withdrawals at DB, DR and ÖBB took place in 1962, 1969 and 1968. The last ones in Poland, designated as Ty43 there, retired from service in 1995.