My dad was 30 years USAF and started out as a tail gunner in B-25's. At high altitude it would routinely get to -50 in those compartments. All they had was the thermal and thick woolen undergarments to help keep their body temperature up. I believe the B-29 was the first bomber to finally get pressurized cabins, but not sure if the gunner compartments had them.

Life was so different. His dad mined coal his whole life in Southern Illinois. I remember when visiting as a young boy him coming home from work solid black.