With its 20-tonne axle load, the class 01 could not be used on many lines. Although the plans provided for the upgrading of all main lines for these weight, this project could not be implemented due to the economic crisis, and it could not have been completed within a few years. Thus, the class 03 was developed, which was almost identical in construction to the 01, but lighter.
The main changes included a bar frame that was now only 90 mm thick instead of 100 mm, a diameter of the cylinders that had been reduced from 650 to 600 mm and the pressure of the slightly smaller boiler that had been reduced to 14 bars. An axle load of 17.7 and later 18.1 tonnes was achieved with almost identical external dimensions, thus significantly expanding the range of lines which could be used. The boiler pressure was increased to 16 bars after the first three locomotives, analogous to most standard locomotives, but in return the diameter of the cylinders was further reduced to 570 mm.
As with the 01, the top speed was initially 120 km/h and was later increased to 130 km/h from 03 163 after the diameter of the leading wheels had been increased. Since the type had even been designed for 140 km/h, some could also be approved for this speed after modifying the braking equipment. With a few, performance at high speeds could be significantly improved by using a complete or partial streamlined fairing, but this was later removed again, as was the case with the development 0310. The production locomotives could haul trains of 430 tonnes at 120 km/h and 790 tonnes at 100 km/h on the level.
The three driving axles were fixed in the frame and only the middle driven axle had wheel flanges which were 15 mm thinner. The front bogie could be moved sideways by 100 mm, the trailing axle behind the firebox was an Adams axle and had 140 mm of play. As with its heavier sister, the wheels of the bogie were 850 or 1,000 mm depending on the year of manufacture, the coupled wheels were 2,000 mm and the wheels of the trailing axle were 1,250 mm.
With a total of 298 locomotives built between 1930 and 1938, the 03 reached a larger number than the 01. After the Second World War, the West German Bundesbahn received 154, the East German Reichsbahn 86 and the Polish PKP around 40. In the case of the latter, they were called Pm 2. In 1968, only 45 were still in use on the Bundesbahn, all of which were in storage by 1972. The Reichsbahn equipped damaged locomotives with a welded firebox and feed water heaters. The larger part of 55 locomotives was even completely reconstructed and provided with completely welded new boilers between 1969 and 1975. Due to the combustion chamber, they only had a tube length of 5,700 mm instead of the 6,800 mm of the original boilers, which was already in the problematic area. They had an output increased by around 100 kW and were still used in the 1980s.
