After the positive experience with class D.343, in 1970, Italian state railways, Ferrovie dello Stato, ordered further 70 diesel locomotives with some minor improvements.

The locomotives were manufactured by FIAT, Breda and SOFER, with electrical equipment provided by Magneti Marelli, TIBB and Italtrafo.

The central compartment, housing the engine, the generator and the cooling system, is missing the baggage room, which had been never used in the previous class.

The electric generator is a DC one produced by Tecnomasio Italiano Brown Boveri (TIBB), with a power of 750 kW (1,010 hp) .

The command circuit includes electro-pneumatic switches and relays, located in a large cabin in the front cab.