Introduced in 1939 by DWK the model RT125 went on to become the most widely copied motorcycle in the world after World War II. It's original factory was in the Eastern Zone of Germany and was produced there as a DKW. The DKW company resumed production in West Germany and call the model the RT125W to distinguish it from the East German model.

In England BSA produced the RT125, in Moscow it was made as the Minsk M1a, in the USA Harley-Davidson copied it as the Model 125, and in Japan it became the first Yamaha the YA-1 Red Dragon. There were also copies made in Poland and China.

The RT125 was the most popular dispatch rider motorcycle and achieved over 100 miles per gallon on highways. It was far better then the much heavier RT350 due to the poor quality of roads.

As part of the treaty to end the war all German motorcycle plans were ceded to the allows and their patents were voided. The RT125 has been the result of the research by Charles Day, who developed the Day two cycle engine it used. This design was improved greatly by Adolf Schnurle from Deutz who developed a superior flow pattern for the Day engine called Schnurle Reverse Flow. This dramatically improved fuel charging ability and removed the necessity for using a high domed piston. DKW obtained the exclusive license to use the reverse flow design and owned it's patent.

In the 1950s Walter Kaden of MZ (the later name of the East German DKW) improved the Schnurle dsign further by adding more ports which made the original DKW designs very successful in racing when coupled with the expansion chamber.

DKW also invented the Front Drive automobile, and is today a part of Volkswagen. Today's MZ RT125 is the current evolution of the motorcycle that Harley-Davidson copied from DKW.