By 1939, all the main Italian aircraft factories had begun designing a new series of monoplane fighter aircraft, using inline engines as opposed to the radial engines that powered the first generation Italian monoplane fighters used in the early years of World War II (fighters such as the Fiat G.50 and the Macchi C.200). This process saw the first generation radial-engined fighters re-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 (which was an aerodynamically revised Macchi C.200- also known as Macchi C.201 - with an inline V-12 instead of a radial engine). Aircraft in this series were given alphanumeric designations ending in the number "2". However, the process didn't stop, and already in 1941, designers shifted their attention on the new, larger and more powerful Fiat RA.1050, a license-built copy of the Daimler-Benz DB 605. Aircraft powered by this new engine became the "Serie 5", and all had alphanumeric designations ending in the number "5" (Macchi C.205, Reggiane Re.2005, Fiat G.55). 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 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 with 200 rounds of ammunition, installed in the forward fuselage and firing between the cylinder banks, exiting through the propeller hub. In "Sottoserie O" airframes, there were also four 12.7 mm  Breda-SAFAT machine guns; two in the upper engine cowling, and two in the lower cowling, firing through the propeller arc, with 300 rpg. 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 (for a total of three cannon and two 12.7mm machine guns, although this varied; some had machine guns in the wings instead of cannon).

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, license-built 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 , 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 technically advanced, intricate, and therefore time-consuming of the three to produce, which made it unattractive at that stage of the war. The G.55 was chosen for mass production, along with the C.205. The G.55 prototype reached 620 km/h , fully loaded, and without using WEP (war emergency power), at 7,000 m . This was a little less than expected, but it had a strong airframe and was the best aircraft regarding handling and stability at every altitude. The only negative assessment noted by G.55 pilots was the pronounced left-hand yawing at takeoff due to the powerful engine torque. 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 , the typical altitude of the bombers and its light armament of two 12.7 mm  and two 7.7mm  machine guns was hardly adequate to bring down heavy 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 centerline cannon as opposed to 120 rounds in the Re.2005) standardized in the production Serie I, was sufficient to bring down US heavy 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  Breda-SAFAT machine guns, plus two underwing hardpoints, allowing it to carry either two bombs (up to 160 kg), or two drop tanks (100 l ). 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 officer, 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 identification number VF204 applied, was put in the depot at Ford; its final fate is unrecorded.

From that date on, the Centauro served with the Aeronautica Nazionale Repubblicana (ANR), the air force of the new fascist state created in North Italy by Mussolini, with the assistance of the Germans. 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 (possibly as many as 42, according to some official reports) were requisitioned by the Germans.

The Fiat factory, in Turin under German control, continued production for about six months. On 25 April 1944, Fiat factories were heavily bombed: 15 G.55s were destroyed, as well as some trimotor Fiat G.12 transports, BR.20 bombers, and CR.42LW biplane fighters ordered by the Luftwaffe. 164 "Centauros" had been completed, 97 of them being produced after the Armistice and delivered to the ANR. Following the advice of Rüstungs 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.
Type Single seat fighter
Engine 1 Fiat RA.1050 R.C.58 Tifone with a 3-bladed constant speed propeller
Dimensions Length 9,37 m , height 3,13 m ,  span 11,85 m , wing area 21,11 m2  ,
Weights Empty 2630 kg, loaded 3520 kg , max. take off weight 3718 kg 
Performance Max. speed 623 km/h , cruising speed  , range 1200 km, ferry range with 2 100 l drop tanks 1650 km endurance  , service ceiling 12750 m  , climb to 6000 m 5 min. 50 sec., to 7000 m 8 min. 34 sec.
Armament 1  20 mm   MG 151/20 cannon, engine-mounted {250 rounds)
4  12.7 mm  Breda-SAFAT machine guns, two in the upper engine cowling, two in the lower cowling/wing roots (300 rpg)
Type Werk.Nr Registration History
M.M.91068
M.M.91064
M.M.91065
M.M.91066