The scream was recorded in 1951 for "Distant Drums," a western directed by Raoul Walsh and starring Gary Cooper as a captain who leads a group of soldiers on a mission to defend 19th century Florida settlers from Seminole Indians.
During one scene, a soldier is bitten and dragged underwater by an alligator and he screams the whole way down. The soldier may have died, but a new star was born.
After "Distant Drums," the scream stayed quiet for a couple of years until 1953, when a soldier named Pvt. Wilhelm (played by Ralph Brooks in "The Charge at Feather River") got shot in the leg by an arrow. Sound editors needed a good scream and decided to reuse the one from "Distant Drums."
AICN now has a couple of compilations of Wilhelm screams from movies ranging from Star Wars to Howard the Duck.
Update: Okay, here's one of the compilations, but it has one clip in it that has me completely mystified, at the 1:37 mark:
That's Harrison Ford and Chewbacca, and a real Storm Trooper, but I have no idea where that scene is from. Some lost TV special, maybe?
Help me, readers, you're my only hope . . .