The electrification of the Gotthard Line including its approach routes from Lucerne to Chiasso was completed on May 18th, 1922. The locomotives purchased for it (the classes Ce 6/8 II & III, Be 4/6 and Be 4/7) functioned extremely well and clearly showed the advantages of electrical operation.

However, the new electric motive power soon meant increasing loads and the situation on the Gotthard became increasingly tighter. In neighbouring countries, there had soon been talk that loads over the Gotthard could be moved quite a bit faster than previously. Even though the electric locomotives of the first generation were clearly more powerful than the last steam locomotives, they could not take on endlessly increasing loads. A new locomotive class should therefore provide a remedy. The first studies showed that a locomotive with four sets of driving wheels would represent the most favorable solution. Express trains of 600 metric tons would have required two of these units in multiple unit control. There were no multiple unit control for locomotives with electric brakes for this power class, in which the units could be uncouples and coupled again as often as desired.

The Swiss Federal Railways (SBB) therefore decided on a double locomotive that was not separable in operation. This enabled a savings in two engineer's cabs and various pieces of equipment only required once. In February of 1929, the SBB requested the locomotive builders to come up with designs for the desired unit with a considerable specification profile. It had to be 100 km/h fast and able to haul 600 metric ton passenger trains at 62 km/h and 750 metric ton freight trains at 50 km/h on a 2.7% grade. Electric regenerative brakes had to be able to stop the locomotive weight on a 2.7% descending grade. After twenty submitted projects, the SBB decided on the construction of one each prototypes from designs by SLM/BBC and SLM/MFO.