The XO-60 represents the pinnacle of the U.S. military's unsuccessful association with the autogiro during the 1920s and 30s. By the beginning of World War Two, the practical helicopter had begun to appear. Its rotary-wing antecedent, the autogiro, began to fade into oblivion. In the United States the two primary manufacturers of autogiros, Pitcairn and Kellett, embattled survivors of the Great Depression continued to seek government contracts to sustain their businesses. Both remained obstinate in their belief that the autogiro, a proven technology, would be of far more use to the military in the forthcoming conflict than the largely experimental and delicate helicopters then under development. However, when the U.S. Army Air Force took delivery of the Kellett XO-60 in 1944 it was the last of the autogiros - the helicopter having made the type obsolete.