Yes, they did, if you count Merchant Marine, but that's only 240,000 or so, so make that 16.25 million in uniform. See my post #17.
Today we have about 3 million in the military, but half of them are in the reserve forces, that is the Army, Navy, Marine and Air Force Reserves, along with the Army and Air National Guard. Of course that is out of a much larger population, 300M+ today, verses about 137M in the middle of WWII.
That may actually overstate current numbers other sources say 1,454,515 total active duty, but only 848,000 reservists, as of Feb, 2009. That actually sounds about right. We had over 2M active duty alone at the end of the Reagan years.