The SNCASO SO.4000 was an experimental French twin-engine jet-bomber aircraft of the 1950s. Only a single example was built, which only made a single test flight before development was abandoned.
The French Air Force drew up a requirement for a jet bomber shortly after the end of the Second World War, with the new bomber expected to have a weight of about 25–30 tonnes and to fly at high-subsonic speeds. Designs were tendered by SNCASO, the SO 4000 and by SNCAC, the SNCAC NC 270, which resulted in SNCASO receiving an order for two manned scale models, the first, the SNCASO M.1 - an unpowered glider which would be tested from atop the first prototype of the Heinkel He 274 which had remained in France post-war and restored to flightworthiness by the French, while the SNCASO M.2 was powered by a single Rolls-Royce Derwent, and a full-size prototype, as did SNCAC.
The SO.4000 had a mid-mounted wing swept at an angle of 35 degrees, and had a carefully streamlined oval section fuselage accommodating two 22.2 kN (4,980 lbf) Rolls-Royce Nene engines mounted side-by-side in the rear fuselage, while it was fitted with a tall tricycle landing gear with tandem mainwheels. The crew of two sat in a pressurized cockpit in the extreme nose of the aircraft. The aircraft was designed to carry a bombload of up to 5,000 kg (11,000 lb), while it was planned to fit remotely controlled barbettes carrying two 15 mm cannon on the wingtips.
SNCASO (Société nationale des constructions aéronautiques du sud-ouest, or commonly, Sud-Ouest) was a French aircraft manufacturer, which originated on November 16, 1936, from the merger of the factories of Blériot of Suresnes, Bloch of Villacoublay and Courbevoie, SASO (Société Aéronautique du Sud-Ouest) of Bordeaux-Mérignac, UCA (Usine de Construction Aéronautique) of Bordeaux-Bègles, Société Aérienne Bordelaise (SAB) of Bordeaux-Bacalan and Lioré et Olivier of Rochefort. Additionally, SNCASO built a factory in Déols in 1936.
SNCASO took over SNCAO's assets in 1941.
On March 1, 1957, SNCASO merged with SNCASE (Société nationale de constructions aéronautiques du sud-est), to form Sud Aviation.