The US Army Air Corps placed an order for 18 P-6s in 1929. This aircraft was yet another modification of the basic P-1; in this case the P-1B. The P-6 series of aircraft had 13 distinct sub-types of 8 different models (A-H) (although the C model conversion was cancelled before completion). The most famous of the series was the E model, originally designated as the Y1P-22, but changed to P-6E because of the similarity to the P-6 series.