A real mixture of aircraft took part in the Spanish Civil War, representing a combination of:
The non-intervention agreement of 1936 meant that military aircraft could not be openly supplied to either side by or through sympathetic countries (although this had its major impact on the Republicans), so that many aircraft supplied were either non-military, or had to be sent by clandestine means. The Republican government in particular had to purchase aircraft when and where it could, resulting in small numbers of many different models being purchased and used.
The large scale use of aircraft during the first world war led directly to a rapid development in aircraft technology in the interwar years. Not only was it quickly realised that air warfare would be a vital part of any future conflict, but the availability of relatively cheap ex-military aircraft after 1918 led to a boom in interest in civil aviation, both for passenger/goods transport and for pleasure.
The first commercial airlines were set up immediately after the first world war, and have continued to expand ever since. Passenger and cargo planes grew ever bigger, and there were commercial pressures to increase speed and efficiency, leading to the development of a new range of monoplane airliners in the 1930s with very clean lines and retractable landing gear, typified by the Boeing 247 and Douglas DC-1. Of particular significance for the aircraft of the Spanish Civil War was the passing of a law in the USA that every commercial passenger plane had to have at least two engines, so that if one failed the airliner could still fly. This meant that at the start of the Spanish Civil War many American airlines had serviceable, but unusable single-engined aircraft that they needed to get rid of. This is the reason that several examples of types such as the Lockheed Vega, Consolidated Fleetster and Vultee V-1 could be obtained by Republican purchasers relatively easily, if not cheaply.
Pleasure flying probably reached its peak in the 1930s, with the mania for racing and record-breaking. This led to rapid developments in aircraft speed, aerodynamics and construction techniques. Winning air races or setting records was seen by many aircraft manufacturers as the best way to impress potential customers, while at the same time aviators became celebrities.
An example of a typical air event of the 1930s was the MacRobertson Air Race of 1934, between Mildenhall (England) and Melbourne (Australia). This was open to aircraft from many different categories, and attracted 64 entrants from 13 countries, although only 20 participants from 6 countries actually took off. For more details, see http://www.dc3airways.com/1934-1.html.
Interestingly, of the 20 entrants, 5 ended up participating in the Spanish Civil War, as did the Bellanca 28-70 "Irish Swoop", which was scratched from the race for being over weight at the last minute.
A page from Arthur Mees' "Childrens' Encyclopaedia" from 1938
Maybe coming one day...
Maybe coming one day...