wuzak
Captain
But the P-39C was a domestic model and it had the gas heater. @Shortround6 also provided pilot manuals for the D-1 and D-2 which also showed they had the gas heater.
Please provide a source for your assertion that the D had ducted air heating.
I would like to know how much this gas heater weighed.
It must have been in the hundreds of pounds, otherwise the British would not have specified it.