During its short operational service, mostly under the Repubblica Sociale Italiana insignia, after the 8 September 1943 armistice, this powerful, robust and fast aircraft proved itself to be an excellent interceptor at high altitude. In 1944, over Northern Italy, the Centauro clashed with British Supermarine Spitfire, P-51 Mustang, P-47 Thunderbolt and P-38 Lightning, proving to be no easy adversary. Italian fighter pilots liked their Centauro but by the time the war ended, fewer than 300 had been built. (This is in comparison with, for example, the 34,000 Bf 109s built by the Germans.)
By 1939, all the main Italian aircraft factories had begun designing a new series of fighter, with inline engines as opposed to the radial engines that powered the Italian fighters in early World War II. This process brought to the first generation of Italian fighters equipped with the Italian-built copy of the Daimler-Benz DB 601 engine, the so-called Serie 1/2, whose most prominent representative was the Macchi C.202 Folgore. However, the process didn't stop, and already in 1941, designers shifted their attention on the new Daimler-Benz DB 605. Fiat designer Giuseppe Gabrielli, while experimenting a new version of his Fiat G.50 fighter, equipped with the DB 601, started a new design that was to be powered by the Daimler-Benz DB 605.
The first G.55 prototype flew on 30 April 1942, piloted by commander Valentino Cus, immediately showing its good performance and flight characteristics. It was armed with one 20 mm MG 151/20 cannon, installed in the hub with 200 rounds, and four 12.7 mm (.5 in) Breda-SAFAT machine guns, two in the upper engine cowling and two in the lower part, with 300 rpg, in "Sottoserie O" airframes. This layout soon proved to be troublesome, both for rearming and for the servicing of the lower cowling mounted machine guns: for this reason, the two lower machine guns were removed, and replaced with a 20 mm MG 151/20 in each wing, in the later production series, the Serie 1.
The prototype flew to Guidonia, where it was put into trials against the other fighters of the so-called Serie 5 Macchi C.205V Veltro and the formidable Reggiane Re.2005 Sagittario, all of them built around the powerful Daimler-Benz DB 605 engine. The trials showed that the Centauro was the 2nd best performer overall, and it won the tender set by the Regia Aeronautica. The C.205V was good at low and medium altitudes, fast and with good diving characteristics but its performance dropped considerably over 8,000 m (26,250 ft), particularly in handling. The Re.2005 was the fastest at high altitudes and best in dogfights, but suffered from a vibration which turned out to be a balance problem, this was corrected, but was still the most time consuming and technically advanced of the three to produce. The G.55 was chosen for mass production. The G.55 prototype reached 620 km/h (390 mph) full loaded without WEP (war emergency power), at 7,000 m (22,970 ft), a little less than expected, but had a strong airframe and was the best one regarding handling and stability at every altitude. The only negative assessment noted by G.55 pilots was the pronounced left-hand yawing at takeoff. This was partially remedied by a slight offset positioning of the vertical stabilizer to counteract engine torque.
By early 1943, increased Allied bombing raids over Italy had showed that there was no suitable high-altitude fighter to deal with them effectively. The Macchi C.202's performance decreased above 8,000 m (26,250 ft), the typical altitude of the bombers and the MC.202's armament of two 12.7 mm (.5 in) machine guns was hardly adequate to bring down large bombers. Of the Serie 5 fighters, the Centauro showed the best high-altitude performance, due to its large wing surface area. Also its powerful armament, along with the generous ammunition supply (the G.55 had 250 rounds of 20 mm ammunition in the hub cannon as opposed to 120 rounds in the Re.2005) standardized in the production Serie I, was enough to bring down the US bombers.
The Regia Aeronautica commissioned the production of 1,800 G.55s, later raising that number to 2,400. A pre-production series of 34 examples was ordered: these aircraft were mostly based on the prototype, with minor changes to improve its flying characteristics. They had a different weapon layout, as stated above, with the two lower cowling machine guns moved into the wings. Only 19 of the 34 commissioned aircraft were built, and six of them were converted to the Serie I standard at the factory.
The production version, named Serie I, had the standard armament of three 20 mm MG 151/20s and two 12.7 mm (.5 in) Breda-SAFAT machine guns, plus two underwing racks to bring either two bombs (up to 160 kg/350 lb) or two drop tanks (100 L/26 US Gal). At the date of the Armistice, 8 September 1943, 35 G.55s of all Series had been delivered, including three prototypes. Of these, only one was flown to South Italy to join the Italian Co-Belligerent Air Force (a second G.55, MM.91150, was obtained by the Allies in summer 1944, when test pilot, Serafino Agostini, defected with an escaped British POW, an RAF captain sitting on his knees. The aircraft was then taken on charge by the RAF and transferred to the Central Fighter Establishment of Tangmere, Great Britain, on 17 March 1945, with the number VF204 applied, was put in the depot at Ford, then nothing was known anymore of it.
From that date on, the Centauro served with the Aeronautica Nazionale Repubblicana (ANR), the air force of the fascist state created by Mussolini with the Germans' help in North Italy. It still not exactly known how many "Centauros" were eventually requisitioned by the Luftwaffe or those acquired by ANR. About 18 aircraft were expropriated by the ANR while 12-20 (but according to some official reports, 42) were requisitioned by the Germans.
The Fiat factory, in Turin under German control, continued production for about six months and when on 25 April 1944, Fiat factories were heavily bombed (15 G.55s were destroyed with some three-engined transport G.12s, BR.20 bombers and CR.42LWs ordered by the Luftwaffe), 164 "Centauros" had been completed, 97 of them being produced after the Armistice and delivered to ANR. Following the advice of Rustungs und Kriegsproduktion Stab (RuK), the German Control Commission, production was dispersed in small cities of Monferrato and production of parts were assigned to CANSA of Novara and AVIA in Vercelli. The parts were then assembled in Turin where the aircraft were to be flown by test pilots Valentino Cus, Rolandi, Agostini and Catella. Production slowed markedly, and was stopped by the German authorities in September 1944. A total of 148 G.55s were delivered to the ANR and, when the factory was captured, 37 more examples were ready, while 73 were still on the production line, in various degrees of completion.