The 666, like the corresponding medium-sized Fiat 626, was produced from 1940 as "Unified" truck: since 1937, by law, the War Department required all manufacturers of civilian trucks to have standard features regarding weight, number of axles, speed and capacity, in order to simplify logistics in case of requisition by the armed forces.

The truck had a Fiat 366, 6-cylinder 9365 cc, diesel engine, which developed 105 hp at 2000 rpm and was characterized by its innovative layout of guides that allowed a quick and simple replacement from the front.

The civilian version, also available with a bus chassis, had enormous success, also being produced after the war as the Fiat 666N7, for a total production of 8000.

After the Armistice of Cassibile 79 NM were produced for the Germans who occupied the Fiat facilities in north Italy.

In 1942 an armoured personnel carrier version was made, called Fiat 665NM protected or shielded, used against Yugoslav partisans first, and then in northern Italy.