I have to ask, where does this question come from?
The US Army that evolved from the 19th Century to 1940 lost its frontier constabulary function and only retained a role as training/leadership cadre for a army to be raised from a national volunteer or larger conscription group of men. The US Army had no organized reserve formations. Its small reserve of 50 to 60 thousand men were mostly officers and some NCOs, who were fill out the cadre of the recruits being organized into a actual field army/s. There was also the state militia, later the National Guard which could be taken into Federal service in a emergency. In that context the Guard became a partially trained and equiped organized reserve in the 1920s, as opposed to the poorly trained and ill equipped state militias of pre 1917.
Without a major war the US Army would probablly remain as it was in the 1920s. A tiny army serving to generate well trained NCOs and officers who would serve as a teaching staff and eventual leaders for a actual army should one be needed. Tied in with this would be the role as a Research and Development organization for new weapons. While the US army was poorly equipped in 1940 it did benefit from R & D put into artillery, small arms, aircraft, communications, ect... over the previous five decades.
All this would certainly continue to the 1950s absent a major war. The primary influences to modernize would be developments in foreign armies. If you poke through the professional publications of the US Army from 1900 to 1940 you find a lot of well written articles examining doctrines, practice, equipment, ect... from around the world. One that caught my eye in the 'Field Artillery Journal' was written by a Captain who spent two years training with the Japanese army artillery circa 1924. Other US Army officers attended schools or training in South American or European armies. ie: Weidermeyer attended the Kriegsacademie in Germany shortly after the Reichswehr was incorporated into the Wehrmacht. The French army probablly saw the most US Army officers between 1919 & 1939. There were of course observers sent to the small wars around the globe. Stillwell as the 15th Regiment intelligence officer spent a lot of time traveling around China observing combat between the warlords.
I've not read in detail the warplans of 1900 - 1940. Plan Black, Orange, Crimson, Red, Tan, Green, White, Blue, ect... The fragments and summaries I have seen do not contemplate large mobilizations. War Plan Orange outlined a initial preparation of two corps for overseas service against Japan. If the war became protracted then there was the idea a couple more corps might be necessary. The war plan for a Mexican war also outlined the initial preparation of two corps for expeditionary service, with the possibility of more if required.
There was on paper a plan for mobilizing a Army of 60+ divisions, as in the Great War. That was not taken seriously and the preparations of 1920-1930 barely allowed the mobilization of forty divisions out of the Regular Army, Reservists, National Guard and recruits. When the US did mobilize in the latter half of 1940 it took approx fifteen months to have 20-25 divisions combat ready, and another 25 formed and partially ready. That was after twelve months of attempts to review and up date plans, internal reorganization and a large peace time budget increase for 1939-40. were mobilization made from a 'cold start' it might have taken 20-24 months to field 20 combat ready divisions.
To best understand the US Army of the 1920s & 1930s I'd recommend reading the biographys of the officers who served in that era. Eisenhower, Bradley, Clark, Stillwell, Krueger, MacArthur, Collins, Weidermeyer,.. The list is large and all had insightful comments on everything including war plans and preparations. The professional journals of the era, like the Cavalry Journal, Infantry Journal, ect... have a enormous amount of information as well.