Development of a Leopard 1 fitted with the 120mm L44 cannon began as a private venture of the Rheinmetall company, hoping that such a Leopard 1 variant would be of interest, not just to the German army due to logistical commonalities with the Leopard 2 and higher performance of the gun, but also to international customers seeking third generation MBT features on older machines.

In the early 1980s, a Leopard 1A1A1 was modified with a thicker cast steel mantlet, suitable for mounting the Rheinmetall 120mm cannon, and additional interior changes to hold the respective ammunition. Further changes were applied in early 1985, with the installation of the EMES 18 fire control system from the Leopard 1A5.

Although there were several different prototypes based on Leopard 1 outfitted with the 120mm Rheinmetall cannon, none of them managed to justify a costly large scale upgrade effort of existing Leopard 1s. The upgrade was largely considered redundant due to the already more advanced Leopard 2 being in service.