11/30/2010. Thirteen Ford Tri-Motors of different types were acquired by USAAC. The first, in 1928, was a commercial 4-AT-B type designated C-3 (s/n 28-348). Seven C-3As (s/n 29-220 to s/n 29-226) were similar with 235 hp Wright R-790-3 engines; all were later re-engined with 300 hp Wright R-975-1s and designated C-9.
A single 5-AT-B type in 1929 was the C-4 (s/n 29-219) followed in 1931 by four C-4As (s/n 31-401 to 31-404) similar to the civil 5-AT-D. Engines were 450 hp Pratt & Whitney R-1340-3 and -11 respectively and one C-4A was re-designated C-4B when fitted with R-1340-7s.