Many countries started issuing airmail stamps as transporting of mail by air started gaining popularity. Postal administrations charged a premium for Airmail service and so special airmail stamps were issued to cover the higher rate.
The honor of issuing the first airmail stamp goes to Italy.
An existing Express Delivery stamp was overprinted and issued on 22nd May, 1917 for the first official airmail between Rome and Turin.
The first airmail stamps were issued in the US on 15th May, 1918 when the first official airmail between Washington, Philadelphia and New York was inaugurated. The basic rate per ounce for transmission by air was 14c "air fee" and 10c for the special delivery fee. For this purpose a 24c stamp depicting the Curtiss Jenny was issued. Since this was a new service the stamp was issued in a distinguishing bi-colored format, in the patriotic colors - red, white and blue.
This stamp has become one of the most famous US stamps as one sheet of 100 stamps was sold with the center inverted and is well known as the "inverted jenny". A single mint (unused) stamps was sold for $977,500 in an auction on 14 November 2007.
As the airmail fee dropped, a 16c denomination was issued on 11 July 1918 and a 6c stamp was issued in December that year when the special delivery fee was dropped.
Issue of airmail stamps became the norm from then on as countries started airmail services within their countries and then to countries round the globe. Not all stamps had any "aviation" motif on them but all bore the word "airmail" "correo areo" "par avion", 'postes aerienne" or similar inscriptions in the language of origin. In most cases all mail sent by air had necessarily to be franked with airmail stamps. Airmail etiquettes (labels) were also issued but letters could be also be inscribed "by airmail" or equivalent as use of labels was not compulsory. To start with Some countries just overprinted existing stamps with the word airmail or a plane.
A few stamps overprinted with airplanes for use on airmail letters.