Airline history in the continental United States of America was shaped early in its life by the US Postal Service's airmail contract system. This was extended to allow passenger carriers to win routes. The first routes were long-haul, trans-continental trunk routes connecting major hubs and these routes were what made the 'big-four' US airlines successful.
Secondary local routes feeding the trans-continental hubs were then awarded. So carriers in the US were divided into major and local companies. In more recent times most of these smaller carriers were taken over by the larger ones and so we see American, for instance, flying both trunk and feeder services.
This site is truly extensive, covering the history of commercial aviation throughout the world. The site gives brief histories of the beginnings and development of airlines in the United Kingdom, America, and Europe. For those interested in airlines in other parts of the world, the contents also include an exhaustive list of profiles of current and past airlines listed alphabetically, from ABA_Swedish Airlines to ZONDA-Zonas Oeste y Norte de Aerolineas Argentinas. There is also an index of airliners from 1910 with images and stats, and features on London's airports, planes that never made it into production, supersonic transport, and the flying boat. The navigation is simple and information is easy to find.