Although there was already a very powerful locomotive for operations on the mountain lines in the Ore Mountains (Erzgebirge) of eastern Germany in the shape of the ten-coupled ex-Saxon Class 99.67-71, there was a further requirement for an even more powerful class. So it fell to the newly formed Reichsbahn railway division of Dresden to procured an Einheitslok with a 750 mm (2 ft 5 1⁄2 in) track gauge. The Standardisation Office of the German Locomotive Union in Berlin-Tegel prepared the design for this class.
The first series of 13 locomotives was built by the Sächsische Maschinenfabrik, previously Richard Hartmann, in Chemnitz. According to the supply agreement, the Sächsische Maschinenfabrik was supposed to deliver more locos, but as a result of its bankruptcy and liquidation in 1930, this order was transferred to the Berliner Maschinenbau AG (BMAG), previously Schwartzkopff. In 1928, seven locomotives were supplied by BMAG and another twelve followed in 1933.
The locomotives were very modern for their time, and were similar in design to the standard gauge Einheitslokomotiven. The engines fulfilled expectations; by double-heading it was now possible to haul even very long (up to 56 axles) narrow gauge trains uphill.
In 1945, ten locomotives had to be given to the Soviet Union as war reparations. At the same time, there was an enormous increase in the transportation required in the Erzgebirge mountains as a result of new uranium mines opened by SDAG Wismut. In 1952, in order to assist with the resulting shortage of locomotives, a similar follow-on class emerged, DR Class 99.77-79, built by the VEB Lokomotivbau Karl Marx in Babelsberg. At the end of the 1960s, the first Class 99.73-76 engines had to be retired due to boiler damage. Ten locomotives were given new, welded boilers and continued to work their original routes. In 1992, number 99 1760 was converted to oil-firing.
These ten-coupled locomotives had carrying axles housed in a Bissel bogie. The fixed third axle acted as the driving axle and had thinner wheel flanges to begin with. After 1945 its flanges were removed entirely to improve curve running still further. The wheelbase was 3,000 mm (9 ft 10.1 in) initially; this was later increased to 4,000 mm (13 ft 1.5 in).