There was only one P-38K-1-LO built. This prototype (42-13558) combined a P-38G-10-LO airframe with more powerful 1425 hp V-1710-75/77 (V-1710F-15) engines, rated at over 1875 hp war emergency power. The engines were housed in nacelles similar to those of the P-38J and driving broader-chord propellers. In order to accommodate the new propellers it was necessary to increase the diamater of the propeller spinners slightly, which affected the top cowling lines and the interface at the oil cooler/intercooler inlet.
Tests of the P-38K were carried out between Feb 24 and Apr 30, 1943. The performance of the P-38K was quite a bit better than that of the production P-38J--in fact its performance was superior to all other fighters then in production in the USA, including the P-51B and the P-47D. Maximum speed at 29,600 feet was 432 mph. At 40,000 feet, maximum speed was 40 mph faster than that of the P-38J. It was expected that maximum speed at war emergency power could be as high as 450 mph. Initial climb rate was 4800 feet per minute, and an altitude of 20,000 could be reached in 5 minutes. Service ceiling was expected to be above 48,000 feet, and range was expected to be increased by 10 to 15 percent. However, the War Production Board was unwilling to allow even a short production suspension in order to retool for the required changes to the engine cowling. Consequently, the P-38K was not developed any further.
There was some talk throughout the war about fitting the P-38 airframe with a pair of Rolls Royce Merlin XX engines. However the War Production Board was unwilling to shut down the Lightning production for the several months it would have taken to retool for the engine swap. As a result, the Merlin project was shelved and no P-38 was ever flown fitted with Rolls Royce Merlin or Packard Merlin engines. So far as I am aware, no P-38s were ever even retrofitted in the field with Merlin engines.