The P 26/40 was an Italian World War II heavy tank (sometimes defined medium tank when compared to tanks of other nations). It was armed with a 75 mm gun and an 8 mm Breda machine gun, plus another optional machine gun in an anti-aircraft mount. Design had started in 1940 but very few had been built by the time Italy signed the armistice with the Allies in September 1943 and the few produced afterwards were used by the Germans.

The official Italian designation was carro armato ("armoured vehicle") P 26/40. The designation means: P for pesante ("heavy"), the weight of 26 tonnes, and the year of adoption (1940). While considered a heavy tank according to Italy's standards, the P26/40 was much more similar in performance to the medium tanks of other nations.

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load allowed by pontoon bridges) tank with a 47 mm gun, three machine-guns and a crew of five, but this was quickly superseded by another 25 tonne design, to be named P26.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 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 (Fiat 262) was eventually tested, even though in the end it was not adopted.

Provisionally called P75 (from the gun's caliber), the first design (whose prototype was ready on mid-1941) was similar to an enlarged 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.

Only a few (between one and five depending on the source) pre-production models were completed in the months 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.

Only 21 P 40s were finished in September 1943 and during the armistice it served in the defence of Rome. The Germans ordered production to continue after the armistice and appropriated completed tanks to the Southern Tank Training Battalion, 10th and 15th Police Panzer Companies, and the 24th Waffen Mountain Division of the SS Karstjäger. The Southern Tank Training Battalion trained units to use captured Italian tanks and had five P 40s in their inventory. The 10th Police Panzer Company served in Russia before redeployment to northern Italy in late 1944 for anti-partisan duties with the 15th Police Panzer Company. Formed in summer of 1944, the 24th Waffen Mountain Division was deployed to Trieste and Udine along the Adriatic coast. While retreating towards Austria in March 1945, they lost several P 40 tanks to Shermans of the British 6th Armoured Division. About a hundred P 40s were used by the German military, of which about 40 were without engines and used as static emplacements at defensive positions such as the Gustav and Gothic Lines.

The tank served also under Italian crew, it was employed in small numbers in the Armored Group Leoncello of the Italian Social Republic, one of the Leoncello's P40 was also captured by the Italian partisan after the revolts in Milan.