I went to the Purple Heart website at www.purpleheart.org where they have the history of the award. It was first awarded as the Badge Of Miliatry Merit by General Washington to 3 soldiers on May 3, 1783. He revised the award several times. Later it was finally designed to reflect his image on the medal itself.
When Harry Truman was president he made the Purple Heart retroactive to all wounded service men who served in any branch of the military during WWII.
I think since you are a battle buff you will find this website interesting.