I have an original U.S. WWII Army Air Force Graflex k-20 Aircraft camera. Is there any way to tell what missions my camera was used on? I do have the serial number for it.
The K-20 was near ubiquitous among U.S. Army Air Force units that did any kind of photographic reconnaissance. You'd literally be looking for a needle in a haystack.
It's highly unlikely you could use the serial number to trace the camera from the manufacturer to a unit that might have used it. It's more likely that you might find the serial number in maintenance logs or inventory records of any specific unit that used the camera. Your biggest problem is the sheer number of units who used the K-20 and the geographic distance between various locations of any records that may have survived.
If a unit used your particular K-20 in action during WWII and if the maintenance or inventory records from that particular unit are still in existence then probably the only way for you to find it would be to manually examine such records of such units line by line looking for your particular serial number. It's highly doubtful that any such records regarding recce cameras have been digitized, and certainly the case that all of them collectively have not survived, much less been digitized. You'd have to use some old fashioned pre-information age shoe leather to do your search.
Folmer Graflex Corporation only made the K-20 under license from Fairchild, who designed the K-20, between the years of 1941-45. If your K-20 is one of the 15,000 or so K-20 cameras branded as a Graflex it certainly was made during the war years. As was the case with much of the wartime industrial production of the U.S. near the end of the war, it is possible your example was produced too late and never left the U.S. before the war ended. Although a totally different category, most of the surviving WWII "warbirds" in the U.S. were examples that were manufactured near the end of the war and never left the U.S. before the treaties were signed. Some were used by training units here in the U.S. at the end of the war and in the following years before they were replaced by newer technology.
Fairchild, who was a major defense contractor that specialized in imaging devices that worked with visible light, radar, and x-ray, continued to sell the K-20 after the war, but they no longer licensed other companies to do so after the cessation of hostilities.