The P-36A had always been underarmed in comparison with contemporary foreign fighters (e. g. the Spitfire and Hurricane), and P-36A Ser No 38-085 had undergone an experiment in which the fuselage guns were supplemented by the addition of a 0.30-in machine gun in each outer wing panel. This installation was successful, and it was adopted for the last 30 aircraft in the original order (Ser Nos 38-181/210). These were redesignated P-36C. The P-36C also featured an R-1830-17 (S1C3-G) engine rated at 1200 hp for takeoff. These changes had been ordered on January 16, 1939, and the P-36C could be distinguished from the P-36A by the addition of cartridge case retainer boxes protruding underneath the wings. Despite the extra drag produced by the underwing cartridge boxes and the increased weight, the increased power of the engine raised the maximum speed of the P-36C to 311 mph, although the range was lowered to 600 miles. Service ceiling was 33,700 feet. Weights were 4620 lbs empty, 5734 lbs loaded. Wing span was 37 feet 4 inches, length was 28 feet 6 inches, and wing area was 236 square feet.