Lt. Dan Cork and Lt. Tom Harrow are agents in the Allied Intelligence Office (AIO).
The AIO is apparently the key intelligence gathering organization in the fight against the Axis Powers (this was before the creation of the OSS). It is this group that sends military personnel in civilian clothing into enemy territory to spy and sabotage as needed, thus making them spies under the Geneva Convention.
Dan Cork is an American. Tom Harrow is British. The two have apparently known each other for some time because they are described as friends and based on their easy patter with each other. Both are in excellent shape demonstrated by the way they look in their night duty outfits.
These outfits are full body suits, skin-tight totally covering the form from head to toe, including the hands. It is explained that the strange apparel was a specially designed black silk clothing. At first glance it would be easy to assume they were costumed heroes but it is made clear this was to keep them from being spotted when they parachute at night over Germany. Even their chutes and lines are blackened.
In the first adventure we have of two soldiers wearing such outfits leaping from a plane to drop into Berlin, the identities of the men was not given except referring to them as Agent 5 and Agent 9 and going by the nickname of Black Wings, a term that fits the costumes. In the remaining four adventures, they go by their actual names and the term Night Devils are given them.
As time goes by, the twosome gets even more specialized equipment including gliders made up of a new material named renalite which holds together the molecules by use of a kind of atomic magnetism. "By the interference of a certain short-wave vibration, this magnetism is interrupted. The result being immediate release of the molecules and the utter destruction of the renalite." (Not sure what that means but it sounds cool, especially in 1941.)
In their last adventure (a two-parter) the duo are put in charge of more men, each wearing the black silk full-body suits and using these renalite gliders head into France for their biggest assignment yet. When it was successfully, completed, no more was heard of the Night Devils.