Back to this tidbit - the USAF memo, dated March 11th 1944 (that deals with perspective P-38K), says that cockpit heating is 'satisfactory' with newly produced P-38Js. The electric gun heater is introduced with P-38J-15, so the hot air formerly used to heat the guns is now directed in the cockpit. Also the 'intensifier tube inlet' is now of increased size; those two changes more than doubled the amount of heat entering in the cockpit. Lockheed is also trying to seal any gaps where leakage can occur. Here, last page talks about cockpit heating.
I don't understand how issues like cockpit heating and windshield defrosting don't get addressed during development prior to wartime service.
Weren't aircraft flown at high altitudes during development?