I am writing a novel that includes 1939 to present. I am interested in what pilots in training first used when they were entering flight school during the mid-late 1930s in USA. Can you advise me?
If you are referring to the first training aircraft flown (Primary trainer) by a cadet in the US Army Air Corps or the US Navy, then you are looking for the Boeing-Stearman Model 75, called "The Stearman", Kaydet, or by the United States Navy (as the NS and N2S). The Stearman was introduced in 1934 and was the primary trainer until the late 40s.
It should be noted the there were other trainers still used throughout that period, by both the military branches and the private aviation schools that were sometimes contracted by them.
In addition this would have been the first of several trainers encountered by a cadet, once the Stearman was mastered a cadet could expect to move up to more advanced trainers.