The Reggiane Re.2000 was designed by Roberto Longhi and Antonio Alessio in 1938. They took inspiration from the contemporary Seversky P-35 which it superficially resembled. The Re.2000 was the first aircraft designed by Reggiane that employed aluminum stressed skin rather than the wooden or mixed wood and metal structures normally used in contemporary Italian aircraft such as the Savoia-Marchetti SM.79 built by Reggiane under license. Reggiane introduced several advanced features: a modern structure, well more advanced than the ones used in Macchi's and other Italian fighters of the time; an elliptical wing, with five spars and integral fuel tanks. Reggiane Re.2000 had no fuselage tanks, but nevertheless, with the entire wing volume devoted to fuel, it had up to 460 kg (640 lt) gasoline, with a 900 1,100 km endurance, far better than Macchis and Fiats. The armament was still two 12.7 mm Breda's (300 rounds each), plus the provision for bomblet-dispensers (spezzoniera).
The Re.2000 prototype's first flight was on 24 May 1939 at Reggio Emilia, flown by Mario De Bernardi,. The Re.2000, with 260 kg fuel (the maximum was 460 kg/640 lt, but RA did not needed it) was quite light: 2,059 kg empty, 2,529 kg loaded. It reached 518 km/h at 5,250 m and 506 km/h/6,000 m, climbed to 6,000 m in 6,5 min, and had 11,500 m ceiling. During test flights the aircraft gave an excellent performance, and on several occasions, it performed better than other fighters then in production. In mock dogfights, it could successfully fight not only the slower Fiat CR.42 biplane, but even the more modern Macchi C.200 and the German Bf 109E.
The Re.2000GA (Grande Autonomia) version, added extra 340 liters fuel tanks. This version should have been used to reach Eastern Africa, but it wasn't ready before the Italian defeat; the only fighter sent to reinforce Regia Aeronautica units were 51 C.R.42s disassembled and transported by the large Savoia-Marchetti SM.82s. Re.2000GAs were heavier and significantly slower than the standard production Re.2000s. At an empty weight was 2,190 kg compared to the Re.2000's 2,080 kg, maximum speed was 520 km/h at 5,300 m. Armament was two 12.7 mm SAFAT with 600 rounds, and provisions for a Nardi dispenser with 88 2 kg bomblets (a typical 'special armament' for Re.2000s). The Re.2000GA was never really reliable (even for Re.2000 standards), especially its troublesome engine.
The Re.2000bis equipped with P XI bis engine was built in small numbers with only nine examples delivered when 377a Sq. was born, in August 1941.