Before Mollo was hired, McQuarrie and Lucas came up with a Darth Vader design heavily inspired by Japanese samurai with a dash of David Lean’s epic desert-based masterpiece “Lawrence of Arabia.” Lucas wanted his villain to have a “fluttering” look that he described as “a dark lord riding on the wind,” which is the “Lawrence of Arabia” connection as the original idea was to dress him in a silky, black outfit. This would later evolve into Vader’s iconic cape.
Vader was also supposed to wear a sort of Japanese kabuto, which is a samurai warrior’s helmet, and McQuarrie thought it would be cool to throw in a samurai face mask (known as a mempo) as well. That ended up evolving into a breathing device, which was an idea once again put forth by McQuarrie. He noticed that Vader’s introduction was him moving from one ship to another and thought it would make sense that Vader would need something to help him breathe during that transition. Lucas dug this idea so much that he wanted it to be a permanent part of Vader’s look.
Keep in mind, this is before Anakin Skywalker’s backstory was put in place, and Lucas would end up folding that into Vader’s backstory.
When Mollo started seeing McQuarrie’s artwork, he immediately noticed the similarities between Vader’s outfit and the look of the German soldiers in both WW1 and WW2. The suit reminded him of trench pieces worn by the Germans in WW1 and he knew the helmet top had to be fashioned after the Nazi combat helmet.