Why is it the 40-yard dash at the NFL Combine anyway?

The 40-yard dash has become the way for scouts to gauge an NFL prospect's speed, and the marquee event at the NFL combine, which began this week.

But why is it 40 yards? Why not the 25, or the 50-yard dash? According to Michael MacCambridge's America's Game: The Epic Story of How Pro Football Captured a Nation, Cleveland Browns coach Paul Brown came up with the number in the mid-1940s, figuring the 40 was a "more meaningful measure of true football speed" than the 50- or even the 100-yard dash, because it was about the distance a player would cover on a punt:

According to Sports Illustrated, Brown used the 40-yard dash as a tiebreaker to figure out who could cover a punt. Then Gil Brandt made it a uniform test with the Cowboys in the 1960s, determining that the 40 was the best distance for skill-position players. Other teams quickly followed suit, and the rest is history.