In 1869 and 1870, some locomotives with the wheel arrangements 2-2-2 and 4-2-0 (Crampton) were converted into four-coupled ones in order to increase the tractive effort for the increased loads in express traffic. Four locomotives with 0-4-2 wheel arrangement were then ordered and assigned to class B IX, but soon sold to the Reichseisenbahnen Alsace-Lorraine. Finally, in 1874, Maffei began to manufacture the new B IX, which had a 2-4-0 wheel arrangement and was thus the first four-coupled express locomotive in Bavaria to be procured in series.

Since at that time engineers had already come to the realization that overhanging masses impair the running properties at high speeds, attempts were made to reduce them as much as possible. With the B VIII, which was built in small numbers, experience had already been gained with a firebox whose center of gravity was directly above the second coupled axle, and this feature was adopted with the B IX. In order to also minimize the front overhangs, the cylinders were moved as close as possible to the leading axle. Like the previous series, the B IX had an outer frame, internal Stephenson valve gear and external cylinders. Due to its smooth running, it could be approved for 90 km/h and, with a train weight of 150 tonnes, reached a speed of 70 km/h on the flat.

A total of 104 engines were made between 1874 and 1887. After the end of the First World War, they were no longer used in high-value express service, but the remaining units with the numbers 34 7421 to 34 7440 were included in the Reichsbahn's provisional renumbering plan. However, the new numbers did not need to be attached, since the last machines were retired in 1925. Today, the locomotive with the number 1000 can be viewed as a cutaway model in the Lokwelt Freilassing, a branch of the Deutsches Museum in Munich.