The Army ordered eight additional C-2's as production versions of the C-2 "Bird of Paradise". These aircraft, designated C-2A, had a slightly longer wingspan than the "Bird of Paradise" but were otherwise very similar.
The last four C-2A's built were modified in an attempt to solve a vibration problem. Because the outboard engines were close to the fuselage, the "prop wash" from the center engine created turbulent air for outboard engines. This in turn created significant vibration and flutter problems in flight.
To solve the problem, engineers initially tried replacing the center engine 2-blade propeller with a 3-blade propeller of smaller diameter.
This helped some but did not solve the problem so the last four C-2A's were modified on the factory floor. The outboard engines on these planes were moved 21" farther outboard, thus eliminating the vibration problem.
The last C-2A built was retained for testing and became the XC-7 after more powerful Wright R-975 radial engines were installed.
The second C-2A built was used for a long duration test flight in 1929.