The development work began in 1940, on Benito Mussolini's specific orders. Initial requirements were for a 20 tonne (the maximum load allowed by pontoon bridges) tank with a 47 mm gun, three machine-guns and a crew of five, but this was quickly superseded by another 25 tonne design, to be named P26. The development work proceeded quickly except for the engine; the Italian military staff, the Stato Maggiore, wanted a diesel power-plant, while the builders favoured a petrol engine.
However, in Italy at the time there were no engines (diesel or petrol) available capable of developing the 300 hp (220 kW) required, and the Italian tank industry (i.e. the duopoly Fiat-Ansaldo) did not turn to easily available aircraft engines for its tanks as the contemporary U.S. and British tank manufacturers had done. The design of a new engine was very slow and in the end a 420 hp (310 kW) petrol engine was eventually tested.
Provisionally called P75 (from the gun's calibre), the first design (whose prototype was ready on mid-1941) was similar to an enlarged Fiat M13/40, but with a 75/18 howitzer (the same fitted on the Semovente da 75/18) and more armour; the prototype was then modified by replacing the main gun with a 75/32 gun with a co-axial machine-gun. After learning about Soviet T-34s in 1941, thanks to a captured tank supplied by the Germans, the whole design was radically modified: the armour was quickly thickened (from 40 to 50 mm on the front and from 30 to 40 mm on the sides) and re-designed, adopting more markedly sloped plates, and the new 75/34 gun was adopted; meanwhile the dual barbette mount in the hull was deleted. The gun designation "75/34" referred to a 75 mm bore diameter gun with a length equal to 34 calibres. However, the weight increase (which now topped at 26 tonne) and the difficulties in finding a suitable engine further hampered the start of mass production; in the end, it was decided that the prototype and the early production samples were to be equipped with a 330 HP SPA 8V diesel engine, later to be replaced by a 420 HP petrol engine.
By the armistice, only a few (between one and five depending on the source) pre-production models were completed before the Italian Armistice in September 1943, at which point they were taken over by the German Wehrmacht. A few were used in combat, under the German designation of Panzerkampfwagen P40 737(i), for example at Anzio; some, without engines, were used as static strongpoints.