Which aircraft was accepted as the first airplane by the Army in 1909?

The correct answer is Airplane No. 1, which was the designation given to the Wright Military Flyer by the U.S. Army. In 1909, after a series of tests, the Army officially accepted this aircraft, marking a significant milestone in military aviation history. The acceptance occurred after the aircraft met the military's requirements, which included a flight duration of at least one hour and the ability to carry a pilot along with a payload. This was an important moment, as it made the U.S. Army the first military force in the world to recognize the military potential of powered flight.

Contextually, the other aircraft listed had their significance but did not meet the criteria for acceptance by the Army in 1909. The Wright Flyer 1 was an earlier model and was not a military designation at that time. The Spirit of St. Louis, famously flown by Charles Lindbergh, was developed much later and is renowned for its transatlantic flight, rather than for military purposes. The Curtiss Pusher, while an important aircraft in its own right, was developed after the Wright Military Flyer and was not the first accepted by the Army. Thus, Airplane No. 1 stands out as the pioneering aircraft in the U.S.