Are 20 inch biceps attainable naturally?
Mine are 18 naturally with minimal body fat and I'm fairly genetically gifted in this area.
Two inches is a lot for arms but I'm by no means the biggest dude around. I'm 6'2 and about 200 pounds here. There are plenty of people who can get 20 inch arms naturally BUT you have to be a genetic freak.
Building arms is mostly about triceps. In terms of biceps though - Most guys waste a ton of time doing dumbbell curls but that's all wrong. Focus on compound lifts like pull-ups and weighted rows. Curls are fine and all but neglecting heavy compound lifts will hurt your gains.