Where do N-numbers come from?
The U.S. received the "N" as its nationality designator under the International Air Navigation Convention, held in 1919.  The Convention prescribed an aircraft-marking scheme of a single letter indicating nationality followed by a hyphen and four identity letters (for example, G-REMS).  The five letters together were to be the aircraft's radio call sign.

In the original 1919 allotment, most of the nations shared first letters.  Only U.S. and four other nations were assigned a unique first letter to be followed by any combination of four letters.  In each case, that first letter was the same as a radio call letter that had been previously assigned to that nation by an evolving series of international agreements.  As of April 1913, for example, Great Britain had complete rights to the radio letters B, G, and M, while sharing certain other letters.  Not surprisingly, Great Britain received G as its aircraft nationality identifier under the 1919 agreement.

During this era, the U.S. had complete rights to the radio letters N and W, and to combinations of K from KDA to KZZ.  Why these particular letters? The assignments of W and K appear to have been arbitrary, according to articles on early radio call signs by Thomas H. White.  In the case of N, Whites notes that the U.S. Navy had used this radio letter since November 1909.

This still leaves the question of why N was chosen over W for the U.S. aircraft identifier.  The answer may lie in the fact that the Government had reserved N for itself, while assigning combinations beginning with K and W to various radio stations along geographic lines.  N would therefore be less confusing as a single national marking for aircraft.

The choice was not universally popular.  The Journal Aviation wanted the U.S. to adopt W in honor of the Wright brothers.  Use of the letter N in the early days seems to have been restricted to aircraft that made international flights.  Compliance was voluntary at this time, since the U.S. did not ratify the 1919 Convention.

No mention of N numbers appeared in the initial Air Commerce Regulations placed in effect by FAA's first predecessor agency in December 1926.  The letter markings that this original set of rules specified were C (commercial), S (state), and P (private), which were to precede the numbers assigned to licensed aircraft.  Unlicensed aircraft had numbers, but no letters, at this time.

The earliest legal requirement for the N marking is found in the first general amendments to the Air Commerce Regulations on March 22,1927.  These amendments mandated that U.S. aircraft engaged in foreign air commerce display the N at the beginning of its identification markings.  Later, this requirement was extended to all U.S. aircraft, regardless of whether they operated beyond the Nation's borders.

A second letter indicating the aircraft's airworthiness category followed the N and preceded the identification numbers.  These airworthiness indicators were; "C" for standad, "R" for restricted, "X" for experimental, and later an "L" for limited, (for example, NC1234).  This was standard until December 31, 1948, when aircraft registered for the first time were required to display identification marks consisting of only the Roman capital letter "N" followed by the registration number.  Existing aircraft operated solely within the United States could continue to display an airworthiness symbol until the first time such aircraft were recovered or refinished to an extent necessitating the reapplication of the identification marks.  After December 31, 1950, all aircraft of United States registry operated outside of the United States were required to display identification marks consisting of the Roman capital letter "N" followed by the registration number.

Until December 31, 1960, the required location for display of nationality and identification marks for fixed- wing aircraft was the wing surfaces, and the vertical surface of either the tail or fuselage.  Effective January 1, 1960, all fixed-wing aircraft were required to display indentification marks on the vertical surfaces or either the tail or fuselage.  Wing surface markings were no longer required.

Current standards for the display of nationality and registration identification marks on U.S. civil aircraft can be found in the Code of Federal Regulations, Title 14, Part 45, Subpart C.

Our thanks to Ned Preston, former FAA historian, who compiled the majority of this information.