It was not in service at the onset of WWII.
Duilio was extensively rebuilt in 1937–1940 at Genoa. Her forecastle deck was extended further aft, until it reached the mainmast. The stern and bow were rebuilt, increasing the length of the ship to 186.9 m (613 ft), and the displacement grew to 28,882 t (28,426 long tons; 31,837 short tons). Her old machinery was replaced with more efficient equipment and her twenty boilers were replaced with eight oil-fired models; the new power plant was rated at 75,000 shp (56,000 kW) and speed increased to 26 kn (48 km/h; 30 mph). The ship's amidships turret was removed and the remaining guns were bored out to 320 mm (13 in). Her secondary battery was completely overhauled; the 152 mm guns were replaced with twelve 135 mm (5.3 in) guns in triple turrets amidships. The anti-aircraft battery was significantly improved, to include ten 90 mm (3.5 in) guns, fifteen 37 mm (1.5 in) 54-cal. guns, and sixteen 20 mm (0.79 in) guns. Later, during World War II, four more 37 mm guns were installed and two of the 20 mm guns were removed. After emerging from the modernization, Duilio's crew numbered 35 officers and 1,450 enlisted men.
Starting on 19 March 1937, she was taken to Genoa where an extensive reconstruction began at the Cantieri del Tirreno shipyard. The work was completed on 15 July 1940, at which point she was recommissioned into the 5th Division of the 1st Squadron, based in Taranto.
