By 1914, when the Army first began to acquire tractor-engined aircraft, the official serial number began to be painted in large block figures on both sides of the fuselage or on the rudder. . These numbers were so large that they could be easily seen and recognized from a considerable distance. At the time of American entry into the First World War, the large numbers were retained on the fuselage and sometimes added to the top of the white rudder stripe. . By early 1918, the letters S.C. (for "Signal Corps") were often added as a prefix to the displayed serial number. When the Army Air Service was created in May of 1918, the letters SC were replaced by A.S. (for "Air Service"). In July of 1926, the Army Air Service was renamed the Army Air Corps, and the serial number prefix became A.C. for "Air Corps".
By late 1924, the fuselage serial numbers began to get smaller in size, until they standardized on four-inch figures on each side of the fuselage. This remained so until 1932. In 1926, the words "U.S. Army" were added to the fuselage number, and in 1928 the manufacturer's name and the Army designation was added to the display. Between 1924 and 1929, it was also common practice to add the manufacturer's name and the Army designation to each side of the rudder, but this was not always done.
The three-line fuselage data block was reduced in size to one-inch characters in 1932 and placed on the left hand side of the fuselage near the cockpit. It is still displayed there in the present day.
Beginning in January of 1942, the serial number of most Army aircraft was painted in eight-inch numbers on the vertical tail (whenever possible). This number became known as the tail number, for obvious reasons. . Since military aircraft were at that time not expected to last more than ten years, the first digit of the fiscal year number was omitted in the tail number as was the AC prefix and the hyphen. . For example, Curtiss P-40B serial number 41-5205 had the tail number 15205 painted on its rudder and Curtiss P-40K serial number 42-11125 had the tail number 211125 painted on the rudder. Since the Army (later Air Force) used the last four digits of the tail number as a radio call sign, for short serial numbers (those less than 100), the tail number was expanded out to four digits by adding zeros in front of the sequence number. For example, 41-38 would have the tail number written as 1038.