Prior to 1919, all aircraft flown by the Army Air Service were referred to by the designation given to them by their manufacturer. During this period, a variety of both domestic and foreign types were operated, with the latter being the primary front-line types during World War I.
In September 1919, the Army Air Service decided that it needed an organized designation sequence, and adopted fifteen classifications, designated by Roman numerals. Several other unnumbered designations were added later. Each designation was assigned an abbreviation, and each design a number within that abbreviation. Variants were designated by alphabetically appending letters to the design number.
AC-1 — designed for "long flights and cross-country work", this was a 169 foot long, 180,000 cubic foot buoyancy craft which had one of its earliest long flights in May 1923.[1]
RN-1 — designated "Zodiac", this was a semi-rigid dirigible, 262.5 feet long with a 360,000 cubic foot buoyancy volume.[2]
Army Air Corps/Army Air Forces/Air Force designations (1924–1962)
Until 1926, the Army Air Service had three sequences for bombers. Light bombers were indicated by the LB- prefix, medium bombers by the B- prefix, and heavy bombers by the HB- prefix. In 1926, the three-category system was scrapped and all bombers subsequently built were placed in the B- sequence.
Beginning with #69, the "M-" (missile) and "B-" (bomber) series diverged. The missiles designated M-69 to M-92, some of which are incorrectly labeled as "formerly designated B-xx" in some sources, never used a "B-" series designation. Beginning with #70, another sequence diverged, the "RS-" (Reconnaissance/Strike) series, which was later changed to the "SR-" (Strategic Reconnaissance) series of the Tri-Service system.
Non-sequential
Some bomber designations were assigned out of sequence.
B-20 Havoc – Douglas (redesignated from A-20 in 1948 after original B-20 was canceled)
B-26 Invader – Douglas (redesignated from A-26 in 1948 after original B-26 was retired)
BLR: Bomber, long range (1935–1936)
A short-lived designation used from 1935–1936 to refer to three long-range bomber projects commissioned by the Army Air Corps. Most of the bombers were night bombers.
C-143 – reserved for what would become the X-19, but never officially assigned and later reused in the Tri-Service system
This sequence was restarted at C-1 with the introduction of the Tri-Service system. However, the original sequence was picked up at C-143 starting in 2005, leading to the US military maintaining two separate sequences for cargo aircraft.
The USAF established a separate sequence for purpose-built research aircraft in 1946. Originally designated with the "S" mission letter, the sequence switched to "X" in 1948.
Designated P- for "pursuit" until June 1948, nine months after the United States Air Force was founded. After this, all P- designations were changed to F- ("fighter"), but the original design numbers were retained.[5]
In 1941, the category letter R- was allotted for "rotary wing" aircraft, and this designation was used until the founding of the United States Air Force in 1947, at which point the category letter was changed to H-, for "helicopter". However, the original numbering sequence was retained.
^"Airship AC-1 Lands At Field". The Airship Log. Vol. 1. Belleville, Illinois: Meyer & Farrell. 4 May 1923. p. 1 – via Newspapers.com.
^"RN-1 Zodiac Arrived at Scott Field". The Airship Log. Vol. 1, no. 1. Belleville, Illinois: Meyer & Farrell. 4 May 1923. p. 1 – via Newspapers.com.
^"Lockheed B-71 (SR-71)". National Museum of the United States Air Force. October 29, 2009. Archived from the original on October 4, 2013. Retrieved 2013-10-02.