The gruppo 691 was created by rebuilding all 33 locomotives of the gruppo 690. Although the 690 had four large cylinders, the boiler could not keep up with their steam demand at speed. So they received the boiler of the gruppo 746 that had, above all, a larger grate. They also got larger axle bearings since the original ones tended to fail at higher speeds. The cylinder size remained exactly the same.

In the result, power increased by 300 hp for a total of 1,750. This enabled 691.011 to set the Italian steam locomotive speed record to 150 km/h (93 mph) in 1939. One year earlier, 691.026 had been streamlined, but rebuilt back in 1943 since the casing did not bring great benefits. All were withdrawn in 1962 and 1963. Only 691.022 was preserved and can today be found in the museum at Milan.