As of 2015, serial numbers on modern U.S. military aircraft are located on the tail section of the plane. Serial numbers originally appeared on the fuselage, but this changed in 1941. Different branches of service use different serial number configurations, and these have changed over the decades.
Until 1958, military aircraft serial numbers were always at least four digits, and sometimes those longer than four were shortened. Pilots used these serial numbers as their radio call signs, and in 1958, call signs were lengthened to five digits. Planes with fewer than five digits in the serial number were assigned extra zeros to fill them out. Since 1969, certain tactical aircraft display a two-letter code and the last three digits of the serial number.