Where does the term "balls to the wall" come from?

When I pushed the throttles for takeoff, my instructor told me to go, "balls to the wall." 

When I pushed the throttles for takeoff, my instructor told me to go, "balls to the wall." 

Not long ago, I was flying an Airbus A320 simulator when I learned something interesting. The phrase, "balls to the wall" is an aviation term. 

This came up as I was taking off from Denver International Airport and was moving the throttles forward for takeoff. My instructor, a Frontier Airlines pilot, explained that the phrase is an old war term. He thought it came from World War II, but some cursory Internet research suggests it probably started a little bit later. 

Here's how Salon described the term in 2006. 

In many planes, control sticks are topped with a ball-shaped grip. One such control is the throttle—to get maximum power you push it all the way forward, to the front of the cockpit, or firewall (so-called because it prevents an engine fire from reaching the rest of the plane). Another control is the joystick—pushing it forward sends a plane into a dive. So, literally pushing the balls to the (fire)wall would put a plane into a maximum-speed dive, and figuratively going balls to the wall is doing something all-out, with maximum effort.

The story suggests the term became popular in the Vietnam War, but it is possible some pilots were using in the Korean War. During World War II, the story says, pilots used the term, "balls out" instead. 

Are you surprised? Or did you already know the origin of the term?