The F.4 had been redesigned to take the German DB 601Aa engine, finally flying with this in late 1940. Results were excellent, but Fabrizi decided not to pursue this, developing it instead with the more powerful DB 605A of 1475 hp, which was to be built in Italy as the Fiat RA1050 RC58 Tifone. With this engine the F.6 flew in 1942, and subsequently an all-metal F.6M was built, passing through various development stages. Basic armament was four 12.7-mm (0.54in) Breda-SAFAT machine-gups. Several developments were projected, most having the X-form Isotta-Fraschini Zeta engine, but the Armistice halted further work in September 1943.