The F.6 design was the result of a project to adapt the airframe of the Italian Caproni Vizzola F.5 fighter with the GermanDaimler-Benz DB 605A liquid-cooled inverted V-12 engine. To accomplish this, the Caproni company retained the F.5 fuselage but designed metal wings to replace the wood wings of the F.5. The new aircraft was designated F.6M, with "F" standing for Fabrizi, the designer of the F.5, and "M" for Metallico. It was designed to carry twice the offensive armament of the F.5, with four instead of two Breda-SAFAT machine guns; the prototype F.6M flew with two of these mounted in the fuselage and provision for two more in the wings, although the wing guns were never mounted. The F.6M prototype first flew in September 1941, using a large radiator mounted under he nose, just behind the propeller. Flight testing showed that this location produced significant aerodynamic drag, and the prototype was reworked to mount the radiator on the belly, aft of the pilot position. The sole F.6M prototype was damaged in a collision at Bresso airfield. After this accident, Caproni decided to end the F.6M project and instead concentrate on the more advanced Caproni Vizzola F.7 fighter.
F.6Z
Shortly after Caproni began to design the F.6M, it began work on a second F.6 prototype, this one designed to use the Isotta-Fraschini Zeta R.C.25/60 24-cylinder X-type engine and designated the F.6Z, with "Z" standing for Zeta. The aircraft was to carry three Breda-SAFAT machine guns, one in the fuselage and two in the wings. Problems with engine development greatly delayed the F.6Z, but it finally flew in August 1943. Testing in August and September 1943 showed that the Zeta engine, although rated at 1,100 kilowatts, was producing only 900 kilowatts, and this problem was not solved before the World War II Italian armistice with the Allies on 8 September 1943. This brought the F.6Z project to an end. The sole F.6Z prototype was the only fighter to be powered by the Zeta engine. One other fighter type, the Reggiane Re.2004 was also planned with this engine, but was not realized.
