Further US high altitude fighters.
YP-37 with turbo, 13 ordered December 11, 1937 first one delivered in June of 1939. Turbo troubles experiences with the earlier XP-37 continued despite a new turbo model.
Turbo troubles plagued the Bell XFM-1 series.
Last few planes on the production line had the turbos removed before completion.
As mentioned above the US was buying B-17s with turbos with deliveries starting in the summer of 1939.
The First A-20 attack bombers were supposed to have turbos but the first plane had such persistent overheating problems that the turbos were deleted on the rest of the production batch and never came back.
Grumman P-50 fighter was lost when one of it's turbos exploded in flight.
At the 1939 Army fighter trials two entrants were powered by P & W R-1830 engines with mechanical two stage superchargers. Predecessor to the engine used in the Grumman F4F.
The USAAC was very interested in high altitude flight and the technology needed for it. However their experience with it in late 30s lead them to believe it was still several years away from being suitable for service squadron use (and that may have been optimistic) so they ordered the highest altitude rated single stage engines they could get for the P-40 and P-39 to get some sort of modern fighters into service as soon as possible.
The idea that the USAAC was not interested in high altitude fighters is a myth.