The Swiss continued development of the MS.412 when French involvement stopped following the June 1940 Armistice. The Dornier-Altenrhein factory completed a prototype powered with a licensed-produced HS-51 12Y engine, generating together with the fixed radiator and revised exhausts as tested on the MS.411, in October 1940. The new type retained the armament changes and other improvements introduced on the D.3800. This series was put into production in 1941 as the "D-3801" with continued deliveries until 1945 with 207 completed. Another 17 were built from spares between 1947 and 1948. Reliability of the new engine was at first extremely poor, with problems with crankshaft bearings causing several accidents. The engine problems slowed deliveries, with only 16 aircraft produced in 1942 and a single aircraft delivered in 1943. The engine problems were eventually resolved in 1944. With from the Hispano-Suiza 12Y-51, the speed was boosted to , roughly equivalent to the D.520 or the Hurricane. Weights were between . After being retired from operational use as a fighter when the North American P-51 Mustang was acquired in 1948, the type remained in service as a trainer and target tug until 1959.