Passage: In early 1937, the USAAC ordered Curtiss to adapt one P-36 to the new liquid-cooled turbo-supercharged Allison V-1710 engine with 1,150 hp (860 kW). Designated Curtiss XP-37, the aircraft used the original Model 75 airframe with radiators mounted on the sides of the fuselage around the engine. The cockpit was moved far to the rear to make room for the radiators and the bulky turbocharger system, and to balance the aircraft. The aircraft flew in April 1937, reaching 340 mph (550 km/h) at 20,000 ft (6,100 m). Although the turbo-supercharger was extremely unreliable and visibility from the cockpit on takeoff and landing was virtually nonexistent, the USAAC was sufficiently intrigued by the promised performance to order 13 service test YP-37s and spares at a contract cost of $531,305.12. Featuring improved aerodynamics and a more reliable turbo-supercharger, the aircraft first flew in June 1939. However, the powerplant remained unreliable and the project was cancelled in favor of another Curtiss design, the P-40.

Question: In what year did the improved aircraft first flew?
1939