EXCERPTS:
On August 1, 1907, the Aeronautical Division of the United States Army Signal Corps was established, and the United States Army purchased its first heavier-than-air aircraft, a Wright Model A, in 1908. It was allocated the serial number 1. Further Army aircraft were assigned serial numbers in sequence of their purchase. Unfortunately, early records from these days are rather incomplete, and there are numerous gaps and conflicts.
To add to the confusion, it often happened that at the time an aircraft was rebuilt, it was assigned a brand new serial number. Some aircraft from this period (for example the DH-4 "Liberty Plane") are known to have carried at least four serial numbers during their careers. After a while, certain serial number blocks were introduced--the 200 block was reserved for seaplanes, the 40000 block for experimental aircraft, and the 94000 block for prototypes and aircraft under evaluation.
On July 2, 1926, the Army Air Service was renamed the United States Army Air Corps (USAAC). On June 20, 1941, the USAAC was renamed the United States Army Air Force (USAAF). On September 18, 1947, the United States Army Air Force was split off from the US Army and became a separate service, the United States Air Force. Throughout all of these changes the earlier fiscal-year serial number system remained unchanged.
EXCERPTS:
Manufacturer's Serial Numbers
When an aircraft is constructed, the company which built it assigns it a manufacturer's serial number. This number is usually displayed on a plate mounted somewhere inside the aircraft. When the aircraft is sold to the Air Force, it is issued a military serial number by the Defense Department. These two numbers bear no relationship with each other, but they are often confused with each other. When I know the manufacturer's serial number of a particular military aircraft, I list it.
If a military aircraft ultimately ends up in civilian hands, it is issued a civil registration number by the owner's national civilian aviation authority. In the USA, these numbers are issued by the FAA, and are known as N-numbers in the USA, since they all begin with the letter N. Typically, the FAA uses the aircraft's manufacturer serial number to track these aircraft. For example, a lot of C-47 Skytrain aircraft ended up in civilian hands after their military service ended, and they are tracked by using their manufacturer's serial numbers.
Missing Air Crew Reports
During World War II, Missing Air Crew Reports (MACRs) were written to record the facts of the last known circumstances regarding missing air crews. First authorized in May of 1943, MACRs were prepared by the unit shortly after the aircrew loss, and they were then sent to AAF Heaquarters where they were filed. The MACRs were numbered in the order of their issuance. Some MACRs were prepared after the war was over, as needs and circumstances dictated.
In addition, some MACRs were prepared at the end of the war to cover losses that took place prior to the introduction of the MACR system. This why some 1942/43 losses have larger MACR numbers than those that took place after May 1943. A list of MACR numbers (along with the aircraft type, the unit, and the date) can be found at ArmyAirForces.com of World War II. Full copies of MACRs can be ordered from the National Archives at National Archives Military Records.