The United States Navy (USN) hoped to use the V-1710 in its rigid airships USS Akron (ZRS-4) and USS Macon (ZRS-5), but both were equipped with German-built Maybach VL-2 engines as the V-1710 was still in testing when the Macon was lost in February 1935 (the Akron having been lost in April 1933). The USAAC purchased its first V-1710 in December 1932. The Great Depression slowed development, and it was not until December 14, 1936 that the engine next flew in the Consolidated XA-11A testbed. The V-1710-C6 successfully completed the USAAC 150 hour Type certificate on April 23, 1937 at , the first engine of any type to do so. The engine was then offered to aircraft manufacturers where it powered the prototype Curtiss XP-37s. All entrants in the new pursuit competition were designed around it, powering the Lockheed P-38, Bell P-39 and Curtiss P-40. When war British Purchasing Commission asked North American Aviation to build the P-40 under license, NAA instead proposed their own improved aircraft design, using the V-1710 in their North American P-51 Mustang.
Question: How many types of aircraft was the engine used in?
Answer: 3