The Chief Recruiting Officer for the Army, Lt. Colonel Harold N. Gilbert came up with the slogan on May 17th, 1941, several months before Pearl Harbor and the entry of the United States in to the war. Gilbert had been tasked to create a slogan that would help increase the recruitment of fliers and aircraft crews.
The slogan was heavily promoted during the rest of 1941 and the balance of the war years.
Patriotic labels of different kinds were printed for use on mail, along with postcards. Posters were printed and the new slogan was used in their sign off by radio stations. Large quantities of various materials were printed up for National Distribution including 900,000 automobile stickers, 100,000 small posters, 50,000 large ones, 50,000 cards, and one million poster stamps for use on mail.
Eventually the slogan evolved to include:
"Keep Em Rolling" , "Keep Em Firing", "Keep Em Cooking" "Keep Em Fighting, and directed at the enemy "Keep Em Running" and "Keep Em Dying". For the Ladies there was : "Keep Em Happy" urging the girls to be true to their soldiers.
Glenn Miller wrote and recorded "Keep Em Flying"
Credit American Philatelist August 2010 Journal.