700C was definitely not an American standard. 700 refers to the diameter of the tire in millimeters, a French unit. 700A, B, D were also things. To achieve a consistent 700mm diameter with various tire widths, these other rims were of varying diameters. Until 1975, 700C wheels were for tubular tires. Then Mavic introduced the Module E rims along with Michelin's new Elan tire. The bead diameter of the 700C version was 622mm. Mavic and Michelin are both French. Prior to the 1975 introduction of the French Module E and Elan combination, 700 clinchers did not exist. The 700C probably owes its popularity to the transition period during which people wanted to run tubular tires on their race bikes, but have the option of using clinchers during the week.
27" clincher wheels were introduced by Dunlop in the 1930's.