On 1 October 1940, the XF4U-1 became the first single-engine U.S. fighter to fly faster than by flying at an average ground speed of from Stratford to Hartford. The USAAC's twin engine Lockheed P-38 Lightning had flown over 400 mph in January–February 1939. The XF4U-1 also had an excellent rate of climb but testing revealed that some requirements would have to be rewritten. In full-power dive tests, speeds of up to were achieved, but not without damage to the control surfaces and access panels and, in one case, an engine failure. The spin recovery standards also had to be relaxed as recovery from the required two-turn spin proved impossible without resorting to an anti-spin chute. The problems clearly meant delays in getting the design into production.