The two main changes of the M.S. 406 were the inclusion of a new wing structure which saved weight, and the fitting of a retractable radiator, located underneath the fuselage. Powered by the production HS 12Y-31 engine, the new design was over faster than the earlier M.S.405 model. Designed to reach speeds of , examples were tested without encountering any difficulty in reaching up to 730 km/h (454mph) when flown in a dive. Armament consisted of a Hispano-Suiza HS.9 or 404 cannon with 60 rounds in the V of the engine and fired through the propeller hub, and two MAC 1934 machine guns (one in each wing, each with 300 rounds). A weakness of the MAC 1934 was its operation at high altitudes. It was found that at altitudes over , the guns had a tendency to freeze. Heaters were subsequently added to the guns for high-altitude use.