Here's a look at the pre-production artwork done for ANH (from Draft 2 onward, naturally).
Ralph McQuarrie's work (may he rest well in the Force) is stunning as always, but I've always wondered about something in his ANH art: namely, he draws Luke and Leia as looking almost exactly alike. They both have shaggy blonde hair, cut more or less like Mark Hamill's in the final film (which is admittedly a short haircut for Leia).
Now, JW Rinzler claims this could be because Lucas secretly told McQuarrie (and no one else) that Luke and Leia were siblings, but that's frankly preposterous. Nobody knew this, not even Lucas himself, until after ESB came out. It's not like he cast Luke and Leia with a family resemblance in mind, after all...
The real answer lies in the history of McQuarrie's production art. He was first hired to do drawings in late 1974, for the second draft script of ANH. His earliest paintings include a rendering of the Mos Eisley cantina scene, featuring a version of Luke with very short blond hair. No doubt this was because Lucas wanted Luke Starkiller to look like his childhood hero Flash Gordon.
Ralph McQuarrie painting of Luke Skywalker in the cantina, second draft
Not long after Lucas had finished writing the second draft, though, he realized it had no major female characters, and considered reworking the plot to make the hero Luke a girl. He told Ralph McQuarrie to incorporate this idea in his next illustrations. McQuarrie duly created several paintings featuring this "female Luke," who (like the male version she replaced) was blonde, though with a shaggy pixie haircut.
It's worth noting that McQuarrie's drawings of Han Solo from the second draft period seem to give him blond hair and a red beard.
Ralph McQuarrie images of Han and female Luke, c. April 1975
When Lucas finally sat down to write his next draft, Luke stayed male, but now Princess Leia (virtually absent in draft two) returned as a major character. What I'd argue happened next is that, faced with the sudden challenge of designing a new character, McQuarrie simply cribbed from his earlier designs. His later drawings of Luke Starkiller have the blond hair seen in his earliest concepts for Luke, albeit now at a shaggier length. But McQuarrie's Princess Leia was also a pixie-cut blonde, because her design was taken from the now-abandoned character of "female Luke."
Additionally, according to ANH costume designer John Mollo, Lucas wanted Leia to resemble Jean Harlow, the platinum blonde Thirties starlet famous for wearing white dresses. Thus, Lucas may have explicitly instructed McQuarrie to draw Leia as blonde, at the same time he decided that she should wear a white gown. (McQuarrie's design notes from one of his meetings with Lucas refer to Leia's dress as a "Madonna outfit.")
Ralph McQuarrie sketches of Luke and Leia, third draft or later. Especially notable are Leia's white gown and blonde hair in the second image.
Jean Harlow, the Blonde Bombshell of Thirties cinema
Of course, these character designs were little more than placeholders, since real physical actors would eventually have to be cast. But Lucas seems to have stuck to his mental image of Luke in particular. In fact, Mark Hamill probably got the nod over runner-up Will Seltzer because, with his blond locks, he looked "more like Luke" than the ethnically-Jewish Seltzer did with his curly dark brown hair.
Speaking of blond protagonists in SW, costume designer John Mollo's art is also interesting in this regard. Mollo's earliest work on ANH seems to have been based on the third-draft script. (One of his drawings features a humanoid Jabba the Hutt and a character named Montross, two space pirates who appear together on-screen only in that draft.)
Mollo's third-draft work includes notable sketches of Luke, Leia, and Ben Kenobi. An early Mollo drawing of Luke dresses him in a costume from one of Ralph McQuarrie's earliest second-draft production paintings (the cantina scene mentioned above, in fact--which was made before "female Luke" had even been conceived). Luke of course here has the short haircut derived from McQuarrie's painting.
John Mollo's early drawing of Luke Skywalker, based on McQuarrie's second-draft production painting
Mollo's drawings of Leia show her as blonde like in McQuarrie's work, but Mollo's Leia has longer hair than McQuarrie's Princess does in most of his sketches. The major exception is McQuarrie's December 1975 painting of the award ceremony on Yavin, in which he gave Leia long flowing blonde locks. Mollo seems to have copied his Leia's hairstyle from this particular concept image.
Ralph McQuarrie's painting of the throne room ceremony at the end of ANH; note Leia's long blonde hair
John Mollo's drawing of Princess Leia with long blonde hair
As for Ben Kenobi, Mollo's earliest design features him in dark pants and a white Japanese-style shirt, with a brown vest (with utility pockets etc.) worn over the shirt. This is in fact essentially the same outfit Luke wears in McQuarrie's cantina painting and in Mollo's early sketches. It might be considered "Tatooine garb," and is notably different from the Jedi-esque robes Obi-Wan wears throughout the final film.
John Mollo's early costume concept for Ben Kenobi
After Mollo had received the fourth-draft script in January 1976, however, he embarked on a new round of costume drawings, thoroughly revising his earlier work. Luke received shaggier hair, and his clothing now became all-white, a style borrowed from McQuarrie's concept art. And Ben Kenobi now was clothed in white Jedi robes, kimono-style, beneath a voluminous brown desert cloak--the look he would sport in the film as ultimately shot.
Mollo's revised costume design for Luke Skywalker
Mollo's January 1976 drawing of Ben Kenobi, now in white Jedi robes.
Incidentally, ANH storyboard artist Alex Tavoularis seems to have bucked the trend of copying McQuarrie's Princess Leia design, and illustrated his Leia with long dark hair, like Dale Arden from the old Flash Gordon Sunday comics.
Alex Tavoularis's Princess Leia from his third-draft storyboards.
Of course, in his work on subsequent SW films, McQuarrie had to update his Leia to look like Carrie Fisher. His Luke required little tweaking, though, given Mark Hamill's great resemblance to the preexisting concept artwork.