Artist Ralph McQuarrie's first drafts of R2-D2 and C-3P0, with the latter looking very Metropolis. Lucas originally wanted the droid to speak like a Brooklyn used-car dealer

Everything below is directly from Brian Jay Jones' “George Lucas: A Life,” which is much recommended. The key point, Padawan, is that Lucas wanted to make this Flash Gordon-type movie because he wanted to watch it, but no one else was making it so he had to do it. But no studio wanted to make it with him because no one else was making it. Get it? Lucas' idea was the opposite of the zeitgeist, and business people only try to tap into the zeitgiest, not what lies beyond the zeitgeist or what the zeitgeist may be missing. Cf., Steve Martin's standup comedy career. Cf., Jackie Chan distinguishing himself from Bruce Lee with comedy. Cf., pretty much anything that shifts the culture. See what everybody is doing and do the opposite. Particularly if the opposite is exactly what you want to do in the first place.

1973-74

• Lucas began the writing process by making lists of names and locations for his fantasy, scrawling Emperor Ford Xerxes XII—a suitably heroic-sounding name—at the top of one of his notebook pages, followed by single names like Owen, Mace, Biggs, and Valorum. ... Luke Skywalker was on the list from the very start, but he was “Prince of Bebers,” while Han Solo was “leader of the Hubble people.” 
• On April 17 [1974], Lucas began writing another treatment, this one titled The Star Wars. This draft contained the dogfight in space that Lucas wanted to see, as well as a more fully realized plot that channeled bits of Flash Gordon and Kurosawa's The Hidden Fortress. Lucas poured everything he had ever loved about the Saturday morning serials into his treatment, with plenty of chases, close scrapes, exotic creatures, and general derring-do. From The Hidden Fortress he borrowed a few key plot points—namely, a princess being escorted through enemy territory by a wise and battle-scarred general and, more important, two bumbling, bickering bureaucrats to serve as comic relief.
• The main character in this [May 1974] draft is a young man named Annikin Starkiller, who trains to become a Jedi Bendu under 70-year-old general Luke Skywalker. There are two droids providing comic relief, one short and squat, the other a gleaming “Metropolis style” robot ... There's a “huge green-skinned monster with no nose and large gills” named Han Solo, a feisty 14-year-old Princess Leia, references to “lazerswords” and Wookiees, as well as to a “tall, grim-looking general”—and relatively minor character—named Darth Vader. And for the first time, one character bids good-bye with “May the Force of Others be with you.” Lucas was still holding on to elements from his first treatment that he liked, including a fight in a cantina, a chase through an asteroid belt, a rescue from a prison, and the concluding awards ceremony.

1975

• In this latest [Jan. 2] draft, Lucas had more carefully fleshed out the concept of the Force—still called the Force of Others in this version—dividing it neatly into a good side called Ashla and a bad side called Bogan. He had also decided that the Force could be intensified through the possession of a mystical Kiber Crystal...
• This time Luke is prompted into action when he receives a hologram message from his brother Deak, who asks Luke to bring the Kiber Crystal to their wounded father, the Starkiller. Luke hires Han Solo, now a “burly-bearded but ruggedly handsome boy”—pretty much [Francis Ford] Coppola as a starpilot—and his copilot Chewbacca (“resembling a huge gray bushbaby monkey”) to take him to Cloud City, where Deak is now being held prisoner. Luke and Han rescue Deak, escape with the Death Star in pursuit, then head for Yavin, where they use the Kiber Crystal to revive the Starkiller. Luke leads an assault on the Death Star—and though he isn't the one to fire the fatal shot that destroys the space station, Luke returns to Yavin a hero, to lead a revolution at the side of his father.
• Lucas realized he had no leading female characters—he had shuffled Leia off to secondary status too quickly—and therefore decided that Luke was now a girl...
• By May, Luke was male again, and Lucas submitted to [Alan] Ladd a new, hastily written six-page synopsis in which he'd added a new character, a mystical old man he had lifted straight out of the pages of Carlos Castaneda's 1968 The Teachings of Don Juan: A Yaqui Way of Knowledge. ... “Old man can do magic, read minds, talk to things like Don Juan,” Lucas wrote in his May 1975 treatment. By the time Lucas completed the next draft in August, the old man even had a name: General Ben Kenobi.
• In fact, by the third draft, completed in August 1975, Lucas had tightened and improved the script even further, moving Luke more firmly to the center of the script as the hero, and making Leia—instead of Deak—the character who gets captured and needs rescuing. Lucas still had the Kiber Crystal in the script but was beginning to realize that pursuing the stolen plans for the Death Star made for a much more interesting story.
• As 1976 approached, Lucas was finishing up his fourth draft, now officially titled The Adventures of Luke Starkiller, as Taken from the Journal of the Whills, Saga I: Star Wars.

1976

• He had a much better handle on the Force at this point, and had wisely decided to remove the Kiber Crystal from the story altogether, making the Force “more ethereal,” he explained, rather “than to have it solidified in a thing like a crystal.”
• In his latest draft, Kenobi survived his lightsaber duel with Darth Vader by retreating through a blast door that slammed shut behind him. That not only left Vader “with egg on his face,” as Lucas put it, but also made the assault on the Death Star little more than a bit of galactic breaking and entering, with Vader as a flummoxed shopkeeper shaking his fist in rage as the heroes escaped unharmed.
• It was Marcia [Lucas] who had put Ben Kenobi's head on the block, pointing out to George that after escaping the Death Star, the old general didn't have much to do for the rest of the film. Lucas had to agree—“the character stood around with his thumb in his ear”—and Marcia suggested that Kenobi be killed in his lightsaber duel and then offer Luke advice as a spirit guide in the final act. 
• “In the end,” Lucas said later, “I really didn't think we were going to make any money at all on Star Wars.”