Fiats portfolio would grow considerably in 1958, and offer a large span of horsepower output, with the introduction of the 200, 300, 400 and 500 series tractors.

To replace the popular but aging 25, Fiat introduced the 300 series.
The 300 was constructed using a heavy box section steel frame. In one end attached to each side of the gearbox and differential casing, in carrying the engine in the middle, and the other end attached to the front axle.

The gearbox was a simple but easy to use six forward and two reverse, and had PTO and locking differential on the rear axle as standard.
The top speed was 22 km/h for the 311R and 312R, and 21 km/h for the 341R. But in 1961 the topspeed would increase to 26 km/h for the 312R.

Fiats new three-point hitch system named "Control-Matic" was introduced on the 300, 400 and 500 series, and was inspired by the revolutionary three point hitch system with draft control, developed and patented by Harry Ferguson in 1926.

Apart from the 312RB, all 300 series tractors used a revised version of the Fiat 25 engine, being a four cylinder, water-cooled, diesel engine of 1.901 cc, now capable of 29 hp at 2.200 rpm, and 108 Nm at 1.500 rpm. It came with a 24 volt system and starter motor as standard.

The Fiat 300 series lasted till 1964, where it became part of Fiats new and streamlined "Diamond-series" of tractor models, and was renamed the 315.