The aircraft flew for the first time on 1 April 1940. Engine cooling problems arose in the initial flights, resulting in modification to the oil cooling ducts. Further modifications were made to the prototype including reduction in the height of the cockpit canopy, revising the armament installation to four 0.5 in (12.7 mm) machine guns in place of the cannon, redesign of the engine nacelles, adding spinners to the propellers, and extending the fuselage forward of the wing. These changes were completed on 15 July 1941.