Did any P40s have some IFR navigation capabilities or were the strictly VFR ?
Thanks to Jimmie Doolittle, there were early gyro instruments available and and these along with primary panel instruments enabled the early fighters instrument capability although quite limited. 4 leg LF Radio ranges and Adcock ranges were available in the United States and it was actually possible for pilots qualified enough in the ART, (and I stress ART as flying a low freq range requires a certain degree of skill
to use these ranges to fly coast to coast on instruments. Going cross country this way was a bit "taxing" in say a P40, but it WAS possible. Military aircraft of the period did have receivers capable of using a LF radio range.
All you needed to fly a low freq range was a LF radio receiver, a headset, and a compass.
The LF ranges were augmented with non directional radio beacons which also supported primitive but usable navigation capability.
Outside the US, early navigation was a crap shoot. Most combat navigation was accomplished with a combination of celestial, dead reckoning, and pilotage.
I learned to navigate on a LF range. Compared to what we have today, I can tell you that the early pilots who flew the mail and pioneered early radio navigation were a GREAT bunch of sticks!