When Dan O'Bannon was writing the script for Alien, he wrote the character of Ripley to be strong and complex. He also originally imagined Ripley being male. When no actor was found who could portray the character as O'Bannon had imagined, Casting Director Mary Selway brought them a woman who had every quality they were looking for. When Sigourney Weaver filled the role of Ripley, the only script revisions were to change pronouns from "he" to "she". Otherwise, who Ripley was and what Ripley did was immaterial to their gender.

Source: Strong Female Characters are Rarely Strong and Barely Characters | The Mary Sue