The turboprop engine offered to provide the power that early jets lacked. However, no manufacturer really achieved a successful turboprop combat aircraft before jet engines improved. A notable failure was the Skyshark, which was a satisfactory airframe design that suffered terribly from problems with the XT-40 engine. In particular, making the gearbox work reliably proved impossible. Other problems included bearing failures and overheated skin around the exhausts. When fighting intensified in Korea, the Navy and Douglas switched their priorities to the proven Skyraider. Development continued, marred by the loss of gearboxes or props on three of the five Douglas test aircraft. Twelve Skysharks were built, but only eight ever received engines. Meanwhile, jets had overtaken the turboprop and by 1954 Douglas was testing the XA4D Skyhawk, destined for over 50 years of service.