The Purple Heart medal as we know it was created in 1932. Is is true that in the early years of World War II it was (very rarely) given for valor, but that quickly dropped out of practice considering that there was the Silver Star and later the Bronze Star, which was created in 1944. For the most part, it has been issued for WIA and KIA.
When the medal was created, World War I veterans were eligible to put a claim in for the award if they had been wounded. The same was also done for the Silver Star, which was also created in 1932.