The other factor in the development of the large piston engine was the advent of high octane aviation gasoline. General Jimmy Doolittle's greatest contribution to aviation was not all his incredible feats of flying it was his role in pushing the development of 100 Octane Avgas in the 1930's.
Post war availability of 115/145 Octane Avgas enabled another increase in horsepower to give enough available power to allow the development of the final (and finest
) generation of intercontinental piston airliners.