The RM ordered the 4-8-0 locomotives 4501 to 4530 in 1901 due to increasing freight volumes on the Giovi Pass. There they were the successors of the 0-8-0 series 4201 to 4500 (the later FS grupo 420) and still had four driving axles, but a two-axle leading bogie to decrease the axle load and to increase guidance in curves. The requirements included that they had to reach a minimum of 20 km/h with a 500-tonne freight train on an incline of 1.6 percent. At the same time, they had to have a top speeds of at least 60 km/h on flatter sections.
20 were ordered from Ansaldo and ten from Breda. They were two-cylinder compounds similar to the 4-6-0 series 3101 to 3200, the later FS grupo 660. The diameters of the cylinders were 540 and 800 mm and both had a stroke of 680 mm. The last driving axle had 20 mm of lateral play to each side to reduce the rigid wheelbase to 3,040 mm. The firebox was wide and shallow, similar to a Wootten firebox. It had two firing doors and the largest grate of any Italian locomotive with an area of 4.8 square metres. Their output was 900 hp, what was sufficient to reach 29 km/h with the required 500 tonnes on 1.6 percent.
With the nationalization of 1905, they became FS grupo 750. As the FS needed more of them, they ordered ten from Officine Meccaniche to be built in 1906. They managed to make them more powerful by using a smaller grate with only 3.5 m², but longer tubes. Output was now increased to 1,000 hp. From 1915 all were upgraded with new boilers which were also used in the classes 470 and 680. This further increased their power to 1,050 hp.
