What if rotary wing aircraft had arrived say in 1940. What would their roles have likely been? Would they have served mainly in the recon/rescue/supply role as they did later in Korea? It's plausable they would have filled the resuce role in the Pacafic and allowed subs to continue their missions of sinking enemy shipping and not pull air rescue ops. I don't think they would of filled an attack role that early however it's possible that role would have been realized by Korea. Interesting.
You may be interested in this previous thread on the subject. http://www.ww2f.com/what-if/23176-what-if-helicopter-development-accelerated-world-war-2-a.html