The "helmet" you're presumably referring to is in fact just a simple leather cap; it doesn't provide any ballistic protection. Unpressurized planes at high altitude can become very cold (well below -30 degrees Fahrenheit) even in the Pacific. Flying caps served other purposes as well besides warming the pilot's head, like providing a better retention system for the oxygen mask and goggles, and in the case of the United States, a system to hold the receivers for the pilot's interphone system.

http://imgur.com/Tk3nRli

You means like the leather flight helmets?