The ARC-170 is based on the Y-Wing fighter from the January 1975 second draft, which had a pilot, a top/rear tail-gunner bubble turret, and a bombardier position. At that point there weren’t X-Wings, so the Y-Wings were the primary Rebel starfighter.
Colin Cantwell’s original Y-Wing concept had the bombardier seats on the bottom of the fuselage, but the ARC-170 has it on top, as with the Y-Wing’s real-world inspiration, the World War II Grumman TBF Avenger.
During the assault on the Death Star in the January 1975 second draft, Luke flies a Y-Wing as pilot, Chewie Antilles (forerunner of Wedge) is the tail-gunner, and C-3PO and R2-D2 are the bombardiers.
(The 1974 rough draft describes pre-Ralph-McQuarrie R2-D2 as “a short multi-armed tripod”, basically a miniature version of the Martian tripods from HG Wells’ War of the Worlds. This idea later influenced the designs of BD-1 in Jedi: Fallen Order and ID-10 in Battlefront II.)
The three- or four-person Y-Wings in the January 1975 second draft are descended from four-person Imperial starfighters in the 1974 rough draft, which are stolen by rebellious Wookees and used to attack an Imperial space fortress orbiting Leia’s home planet. The Wookees also paint the craft in bright Chris-Foss-style colors, kind of like Sabine Wren’s stolen TIE fighter on Rebels.
The 1974 rough draft also features lighter “destroyer-class” two-person starfighters. Leia’s homeworld of Aquilae uses a model with a pilot in front and a navigator in a separate rear bubble, somewhat like the snowspeeders on Hoth in ESB; the Empire uses two-man “stardestroyers” that have pilot & gunner sitting side-by-side, like the cloud cars on Bespin. (Fitting since the cloud city was the Imperial capital city in the 1974 rough draft).
