The Brewster F2A Buffalo was an American fighter aircraft which saw service early in World War II.
 Designed and built by the Brewster Aeronautical Corporation, it was one of the first U.S. monoplanes with an arrestor hook and other modifications for aircraft carriers.
 The Buffalo won a competition against the Grumman F4F Wildcat in 1939 to become the U.S. Navy's first monoplane fighter aircraft.
 Although superior to the Grumman F3F biplane it replaced and the early F4Fs, the Buffalo was largely obsolete when the United States entered the war, being unstable and overweight, especially when compared to the Japanese Mitsubishi A6M Zero.
The Bell P-63 Kingcobra is an American fighter aircraft developed by Bell Aircraft in World War II from the Bell P-39 Airacobra in an attempt to correct that aircraft's deficiencies.
 Although the P-63 was not accepted for combat use by the United States Army Air Forces, it was adopted by the Soviet Air Force.
Donald James Matthew Blakeslee (September 11, 1917 – September 3, 2008) was an officer in the United States Air Force, whose career began as a pilot in the Royal Canadian Air Force and flew Spitfire fighter aircraft, during World War II.
 He then became a member of the Royal Air Force Eagle squadrons.
 He flew more combat missions against the Luftwaffe than any other American fighter pilot.
The Messerschmitt Bf 109 was a German World War II fighter aircraft.
 It was one of the first true modern fighters of the era, including such features as an all-metal monocoque construction, a closed canopy, and retractable landing gear.
 The Bf 109 was the most produced fighter aircraft during World War II, with 30,573 examples built during the war, and the most produced fighter aircraft in history, with a total of 33,984 units produced up to April 1945.
The Vought F4U Corsair is an American fighter aircraft that saw service primarily in World War II and the Korean War.
 Demand for the aircraft soon overwhelmed Vought's manufacturing capability, resulting in production by Goodyear and Brewster: Goodyear-built Corsairs were designated FG and Brewster-built aircraft F3A.
 From the first prototype delivery to the U.S. Navy in 1940, to final delivery in 1953 to the French, 12,571 F4U Corsairs were manufactured, in 16 separate models, in the longest production run of any piston-engined fighter in U.S. history (1942–53).
The Curtiss P-40 Warhawk is an American single-engined, single-seat, all-metal fighter and ground-attack aircraft that first flew in 1938.
 The P-40 design was a modification of the previous Curtiss P-36 Hawk which reduced development time and enabled a rapid entry into production and operational service.
 The Warhawk was used by most Allied powers during World War II, and remained in frontline service until the end of the war.
 It was the third most-produced American fighter, after the P-51 and P-47; by November 1944, when production of the P-40 ceased, 13,738 had been built, all at Curtiss-Wright Corporation's main production facilities at Buffalo, New York.
The North American Aviation P-51 Mustang is an American long-range, single-seat fighter and fighter-bomber used during World War II and the Korean War, among other conflicts.
 The Mustang was designed in 1940 by North American Aviation (NAA) in response to a requirement of the British Purchasing Commission.
 The Purchasing Commission approached North American Aviation to build Curtiss P-40 fighters under license for the Royal Air Force (RAF).
 Rather than build an old design from another company, North American Aviation proposed the design and production of a more modern fighter.
 The prototype NA-73X airframe was rolled out on 9 September 1940, 102 days after the contract was signed, and first flew on 26 October.
Benjamin Scovill "Ben" Kelsey (March 9, 1906 – March 3, 1981) was an American aeronautical engineer and test pilot who brought success in World War II to the United States Army Air Forces by initiating the manufacture of innovative fighter aircraft designs, and by working to quickly increase American fighter production to meet the needs of the coming war.
The Bell P-39 Airacobra was one of the principal American fighter aircraft in service when the United States entered World War II.
 The P-39 was used by the Soviet Air Force, which scored the highest number of individual kills attributed to any U.S. fighter type in the Eastern European theatre.
 Other major users of the type included the Free French, the Royal Air Force, the United States Army Air Forces, and the Italian Co-Belligerent Air Force.
Edwin A. Doss (September 14, 1914January 7, 1996) was an American fighter pilot and commander in the U.S. Air Force during World War II and Korean War.
 Logging more than 4,500 flying hours, Doss flew 573 combat hours and accrued 280 combat missions during his leadership in the South West Pacific Theatre and Korean War.
 For his two-year service as commander of the 35th Fighter Group during World War II, Doss was awarded the Distinguished Flying Cross, Legion of Merit, and the Air Medal.
 He received his second Legion of Merit and the Korean Ulchi medal with a Silver Star for his assignments as commander of the 49th Fighter Bomber Wing and the 3rd Bomber Wing at Kunsan, Korea.
 Colonel Doss’s service has been cited as integral to the development of long-range fighter tactics in the South West Pacific Theater.

Question: Who designed the second most produced American fighter aircraft during World War II?
Answer: North American Aviation