Great print - I think it may be inspired by wearing white for blackouts - and the helmet does resemble a WWII ARP (air raid patrol) helmet which looks like a WW1 soldier's helmet. The thumbs up isn't a motif I have seen used during WWII but if this is early war it could be revived from WW1. The style of the dress looks pre 1942 to me, so that fits. Most of the air raid precautions in the U.S. were done at the very beginning of U.S. involvement, winter 1941/42, by 1943 it was clear there weren't going to be any aerial bombings and ARP got slack. If this dress is English, or the fabric is English, then it could be 1940-41. Wearing white at night during blackouts was recommended to avoid being hit by cars.