Development of the M.S.405 proceeded fairly slowly; testing revealed the need to modify the wing planform and dihedral, while the Chauvière-built propeller was replaced by a Levasseur-built counterpart. The second "M.S.405-2" prototype with a Hispano-Suiza 12Ycrs engine did not make its first flight until 20 January 1937, almost a year and a half behind the first prototype. Outfitted with the new engine, the second prototype was able to attain a speed of during testing. During July 1937, both prototypes were flown to the Paris Air Show. On 29 July 1938, the second prototype was lost along with its pilot. During March 1937, having been suitably impressed with its demonstrated performance, an initial pre-production order was placed for the construction of 16 pre-production prototypes, which were to incorporate the design improvements that had been made upon the previous version.