LowNSlow,
After trawling through a few P-38 websites, the two following explanations appear contradictory:-
"When the plane was in its first production run, a number were supplied to the British RAF, built according to RAF specifications. To simplify the maintenance, both engines (and props) rotated in the same direction, and for a similar reason, the turbochargers were deleted. These two features were probably two of the most important ones that made the P-38 great. The RAF rejected these castrated "kites" and they wound up in the US Army Air Corps as trainers."
"The first P-38 production runs sold to the British did not include a supercharger fitted to their Allison V-1710 engines.The supercharger (turbocharger) was deemed to be classified equipment by the U.S. Government, and was thus restricted from overseas sales. These P-38's performed poorly, resulting in the order for 667 additional P-38s being cancelled by the RAF."