This series of six airmails (Scott C15-20) marks two significant Panama Canal anniversaries—the 10th anniversary of airmail service in the Canal Zone and the 25th anniversary of the opening of the Panama Canal to ocean-to-ocean ship traffic on August 15, 1914. (The first flight was actually on February 10, 1929, with Charles A. Lindbergh in charge of the first airmail dispatch from Cristobal to Miami.) Collectors almost always refer to this series as 'The Panama Canal 25th Anniversary Airmails'. These stamps all feature an airplane of one type or another superimposed on a Panama Canal-related scene. Many of the denominations do not reflect the common rates then in use for airmail matter sent from the Canal Zone.
Large numbers of first day covers were made of all of the stamps in this series. Literally dozens of individuals and firms operating both on the Isthmus and in the United States prepared cachets. First day covers, with or without a commemorative cachet, frequently bear a copy of each stamp, but only one stamp appears on many others. Naturally, the 1-dollar stamp is the least commonly seen on any cover. Some of the cachets are quite rare, and many are truly attractive, particularly those with a photo by retired sailor Walter G. 'Bones' Crosby in California. Collectors delight in acquiring examples of these colorful stamps on cover, especially those with multiples or combinations of the different stamps to foreign destinations or on registered and special delivery letters.
Surprisingly, except for the 15-cent denomination that paid the airmail rate to the United States, large numbers (sometimes more than half) of these stamps were never sold and were burned after their final withdrawal from sale on February 28, 1941.