At the beginning of the war, the Regia Aeronautica (italian air force) had a lot of obsolete planes, most fighters were biplanes. They also had a few monoplane fighter types of excellent maneuverability, but their radial powerplants of ca. 1.000PS didn´t allow at least 530km/h.
In 1941, german DB 601 and later also DB 605 engines arrived and were built under licence for the existing monoplane fighters (some guys even tried to use a DB 601 with a CR.42 biplane), and the results were good, but the speed didn´t reach allied standards.
Further, italian fighters were mostly too weak armed with only two 12,7mm guns. Because the italian industry wasn´t as fitted for war and capable as the ones of its enemies, they tried to solve the problem with a lightweight fighter:
The Ambrosini S.A.I.207.
This one followed after the S.A.I.107, which had less than 540 PS, but reached 560km/h. The S.A.I.207 were mostly built of nonstrategic materials, had excellent handling and maneuverability. Its service ceiling was 12.000m and its armament was typical italian: two 12,7mm Breda MG.
It reached 625km/h with a a 750PS Isotta-Fraschini Delta RC40 V-engine and 2.000 orders came in. They were cancelled after 15, when the advanced S.A.I.403 was in prototype stadium. 3.000 of them were ordered, but only the prototype was finished. The S.A.I.403 had improved aerodynamics and armament.
But the development of these attractive planes ended in october, 1943 due to the armistice with the allies.
My comment:
Appearing in great number since spring/summer 1944, they would have been a problem for the allies. With intact fuel supply and a anti-bomber-weapon like the later german R4M "Orkan" (Hurricane)
rocket, they would have blown the allied daylight bomber offensive over italy. Imagine: 650km/h with only 750PS! Compare this with the biplanes of the beginning 1930´s, which had almost the same power, but less than 400km/h! Even the excellent Heinkel He 100 out of 1938 needed 1.1750PS for only 20km/h more!
Specifications:
Ambrosini S.A.I. 403 italian fighter
Dimensions:
Wing span: 9,80m
Length: 8,20m
Height: 2,90m
Weights:
Empty: 1.983kg
Maximum Take-Off: 2.640kg
Performance:
Maximum Speed: 650km/h
Service Ceiling: 10.000m
Normal Range: 937km
Powerplant:
One 750PS Isotta-Fraschini R.C.21/60 V-Engine
Armament:
four 12,7mm Breda-SAFAT MG or each two
12,7mm Breda-SAFAT MG and 20mm MG151/20 cannon