Easy... The throttle on early war planes were topped with a round knob. When you push the throttle forward, you increase power. Traditionally, there is a firewall that separates the pilot cabin from the engine. Pushing the throttle all the way forward, toward the firewall, gave the plane full power. Hence the term "balls to the wall" for going full throttle.
Of similar circa, the phrase "the whole nine yards" was coined. What is it's origin? Hint, nothing to do with football.