This airmail series was issued during the year 1941 with First Day of Issue being several major stamp exhibitions throughout the year. They were issued to meet existing rates and denominations were, 6c, 10c, 15c, 20c, 30c and 50c all having the same design, only the colors were different.
In 1944 when the domestic airmail rate increased from 6c per oz to 8c per oz an 8c value was added. The 6c value was also issued in booklet form for ease of use by service personnel in the field.
The 6c value was the workhorse of the series. On December 13, 1941 after the United States entered World War II, FDR signed an executive order giving service personnel using APO/FPO addresses to use a 6c per 1/2 oz "concessionary rate". This took effect December 25, 1941 and remained in effect until September 30, 1946. The rate was later extended to Allied forces and contractors serving under the command of US forces.
Further Reading:
"The Transports", GH Davis, US Stamp Society, 1999
Steve B. Davis