[John Beaman]

The P-47 was the happy recipient of a lot of research and experiments by the USAAF with turbo-superchargers (exhaust driven) and was, therefore, ready for high-altitude combat.

The Germans, on the other hand, never anticipated the WWII high altitude combat that resulted from the USAAF’s high altitude bombing campaign. Recall, that the DB 601 engine was developed in the late 19030s when medium altitude combat was the expected norm. The DB engines were not that great over 20k feet, hence the eventual addition of GM-1 to the DB 601 and 605 for high altitude performance and eventually, the make-shift AS types. This was also true of the RR Merlin which did not have effective high-altitude performance until the Merlin 61(?) in the Spitfire IX. Superchargers for in-line liquid cooled engines were much simpler to develop than for radial engines, hence all the work by the US on the turbo superchargers—it was tedious work. The Germans never made that much effort for radials until the Fw 190 experiments which resulted in nothing for the BMW radials. It was simpler, easier and quicker to do the D-9, pending the Ta 152.

Bottom line: the need was not anticipated and money for experiments was always a big issue for the pre-war Luftwaffe. Goering and Hitler wanted a strong air force, now, not a few years down the road.