With the issue of a specification for a successor to the Potez 631 twin-engined fighter in service with the Armee del’Air, P-E Mercier and Jacques Lecarme of the Societe Nationale de Constructions Aeronautiques de Sud-Est (SNCASE) tendered the design of a highly innovative aircraft, the SE 100. Of mixed construction with a wooden wing and a duralumin-covered welded steel-tube fuselage, the SE 100 featured a retractable tricycle undercarriage with a steerable nosewheel and small outrigger wheels retracting into the bases of the endplate vertical tail surfaces. The main wheels fitted right aft, retracting into the tail rather than the wings or engine nacelles as was conventional.

Powered by two 1,030hp Gnome-Rhone 14N 14-cylinder radial engines, the first prototype SE 100 was flown on 29 March 1939 at Argenteuil. Various modifications, including provision of a retractable ventral fin, were introduced as a result of initial flight test, maximum speed being raised from 560km/h to 580km/h as a result. The proposed armament comprised a nose-mounted battery of four Hispano–Suiza HS.404 20mm cannon and a single aft-firing cannon on an electro-pneumatic mount. It was proposed that the crew would comprise two members for diurnal operations and three members for nocturnal missions, and a crawl tunnel was provided between the cockpits. A number of necessary changes were identified during the tests.

On 5 April 1940, during a landing approach, the pitch mechanism of the starboard propeller malfunctioned and the aircraft was destroyed. At this time, assembly of a second prototype had begun, this possessing slightly larger overall dimensions, dispensing with the crawl tunnel to increase fuselage fuel capacity and having an all-metal one-piece wing. Armament was also revised, the forward-firing battery being increased to six 20mm cannon, paired weapons of the same calibre being mounted in a dorsal turret and an additional 20mm cannon being mounted to fire through a ventral hatch.

The aircraft proved to be around 100 km/h faster than the Potez 631, the French Air Force’s current twin-engined fighter, and production was authorized. The Citroen factory in the suburbs of Paris began tooling-up to assemble 300 SE 100s for deliveries to commence at the end of 1940, but the German occupation of Paris prevented final assembly of the second prototype. Proposed variants included the SE 101 and 102 with 1,200hp Pratt & Whitney Twin Wasp and 1,050hp G-R14N-2/3 engines respectively.

SE 100 (first prototype)
Max take-off weight: 7500 kg / 16535 lb
Empty weight: 5520 kg / 12170 lb
Wingspan: 15.70 m / 52 ft 6 in
Length: 11.80 m / 39 ft 9 in
Height: 4.28 m / 14 ft 1 in
Wing area: 33.06 sq.m / 355.85 sq ft
Max. speed: 580 km/h / 360 mph
Cruise speed: 500 km/h / 311 mph
Range: 1300 km / 808 miles