The locomotives of the class E 91 were heavy freight locomotives for use in the mountains, which had been developed on the basis of the E 77 procured from 1924. In contrast, however, the E 91 featured six powered axles, higher power and lower speed for greater pulling power. Like its predecessor, it was a locomotive with a three-part body, which was mounted on only two long bogies. Above each bogie were two motors, which gave their power to the coupled wheels via a common jackshaft. The structure of the bogies was similar to that of the E 77, but instead of the carrying axles there was a third coupled axle. 

The first locomotives were delivered for the Bavarian and Silesian network with Bavarian or Prussian numbers. The Bavarian ones were first designated as EG 5 with the numbers 22 501 to 22 516 and the Prussian ones were EG 581 to 594. A short time later the former became the E 91⁰ with the running numbers 01 to 16 and the latter became the E 91⁸ with serial numbers 81 to 94, although they were almost identical. The only recognizable difference was the gangway doors on the front sides of the EG 5. Further locomotives were built in 1927, which were delivered directly with road numbers E 91 17 to 20. With an hourly output of 2,200 kW, a top speed of only 55 km/h and the high adhesive weight, they were able to achieve very high tractive forces on inclines. At one percent, a 1,200-tonne freight train could be hauled at 35 km/h. When it was used in passenger service, these could be hauled at 500 tonnes on the same gradients at 45 km/h.

In 1929, the Reichsbahn procured twelve more which were supposed to be even better suited for steep sections. Particular attention was paid to the descents, as conventional brakes could easily become exhausted there with heavy trains. For this reason, an electric brake was installed, which had previously been tested on the E 95 02. Although they didn't increase power, they used the latest advances in technology to reduce weight. Despite the newly added braking system, the weight of the locomotives was reduced by over seven tonnes. They were delivered as E 91⁹ with the running numbers 95 to 105 and stationed in the Silesian network.

In the years that followed, the Reichsbahn left the former Bavarian machines in Bavaria, while the Prussian ones also came to other parts of the country, including the southwestern region. Some examples were retired by the end of the war, the remaining Silesian locomotives came to the Eastern Zone a few months after the end of the war. The latter fared like almost all electric locomotives in the east, they were taken to the Soviet Union and returned to the GDR in 1952/53 in poor condition. A planned preparation for the Rübelandbahn was abandoned, since isolated operation with the 25 kV/50 Hz power system was being set up there.

The Bundesbahn had received 23 of the original 46 locomotives and continued to use them in freight service on electrified mountain lines. Between 1957 and 1960 all locomotives of the slightly different types were modernized according to the same pattern. They were redesignated class 191 in 1968, but decommissioning began the following year. This was completed by 1975 and today only 191 099 is in good condition. It is currently in the Augsburg railway park and used to run as a museum locomotive with some parts of the 191 100, which today is sitting only almost complete on the outside.