Was Darth Vader always meant to be Luke Skywalker's father?

The answer

In George Lucas's earliest Star Wars drafts from the early 1970s, Darth Vader was a relatively minor enforcer character, and Luke's father Anakin Skywalker was an entirely separate person - a heroic Jedi Knight who had been tracked down and killed by Vader. The two characters had no family connection whatsoever. The famous twist first appeared in the second draft of The Empire Strikes Back, written by Leigh Brackett and revised extensively by Lucas. In this version, Vader was reimagined as a corrupted Jedi who fell to the dark side during his training, killed most of the Jedi Knights, and was mutilated in a fight with Obi-Wan Kenobi near a nuclear reactor shaft. The decision to merge father and villain into one character transformed Star Wars from a simple adventure into a multigenerational family tragedy.