Of all-wood construction like its predecessors, the S.A.I. 403 Dardo (Dart) featured numerous detail refinements intended to improve combat performance and to facilitate rapid production. The internal fuel and armament capacities were increased, and the resulting loaded weight of 5820 lb. called for an increase in wing area to 155.6 sq. it. in order to maintain the standard of maneuvrability so important to the Italian airman. The vertical tail surface became larger and more angular, while such aerodynamic refinements as a fully-retractable tailwheel contributed to a minor gain in maximum speed of 5 m.p.h. over the S.A.I. 207. Various fuel and armament combinations were tried in order to make the Dardo suitable for different roles. Armament ranged from the two 12.7-mm. machine guns of the light interceptor model to the two machine guns plus two 20-mm. wing cannon of the general purpose version. A third possibility intended for long-range duties employed only the wing cannon, plus two 42 U.S. gal. drop-tanks mounted beneath the wings. The interceptor, lightest of the three versions, weighed only 5459 lb. fully loaded.
3000 aircraft were ordered in January, 1943 to replace the S.A.I. 207 contracts, and production was to be undertaken by Caproni and Savoia Marchetti in addition to the parent firm. However, this promising project came to an end with the Italian Armistice. Similar in concept to lightweight wooden fighters developed by other nations, the Ambrosini designs were the only fighters of the class which demonstrated both practicability and truly exceptional performance.
The S.A.I. 403 had a span of 32 ft. 1 3/4 in., a length of 26 ft. 10 3/4 in., and a height of 9 ft. 6 in. Powered by the 750 h.p. Isotta-Fraschini Delta R.C.21/60 inverted-vee engine, the fighter achieved a maximum of 403 m.p.h. and cruised at 304 m.p.h. An altitude of 19,680 ft. could be reached in 6 min. 40 sec.; service ceiling was 39,810 ft. Range varied from 582 to 1164 miles, depending on the different fuel and armament arrangements.