I was scrolling through old threads and noticed you didn't get a really decisive answer and I had some time on my hands, so here you go.

This one’s a big one.

Tl;dr Essentially yes.

Immediately prior to Pearl Harbor, say, late November 1941, the United States Army, legally called the Army of the United States, had a structure largely defined by the National Defense Acts of 1916, 1920, and 1933;

• Regular Army: The peacetime, voluntary, Army. Per the National Defense Act of 1916, men were permitted to join the Regular Army and enter into active duty for periods of three years at a time, followed by four years of service in a "Regular Army Reserve" component. Men could sign up for as many three-year periods as suited them, until they reached the mandatory retirement age (64). In 1920, the enlistment periods were changed to one or three-year periods, with additional periods of three years at the discretion of the soldier

• National Guard: The reforms by Secretary of War Elihu Root after the Spanish-American War led in part to the Militia Act of 1903, which formed recognized state militia units into what we know today as the National Guard. National Guard units of each state were required to adopt the name "National Guard" (several states had done this already), take on the structure of Regular Army units, and submit themselves to the federal government if they wished to receive funding for training and drills. The situation of the National Guard in war was ambiguous until 1916, when the National Defense Act clarified that National Guardsmen would be drafted into federal service, losing their state militia status and becoming a part of what that act called the "Volunteer Army." Enlistment periods in the National Guard per the act of 1916 were for six years; three years in an active component, and three years in a "National Guard Reserve" component. In 1920, this was altered to an initial period of three years, with additional periods of one year at the discretion of the soldier; from 1933, an initial period of three years, with additional periods of one or three years at the discretion of the soldier. Also in 1933, the National Guard of the United States was formed. Essentially, all members of the National Guard of each state were now simultaneously members of the Army of the United States instead of being an underling of it, further detaching them from their state militia status. The units still weren't under government control until the president authorized them to be, however. National Guard officers were considered reserve officers. On October 21, 1940, states were authorized to create "military units not a part of the National Guard," often called "state defense forces," that were under the control of the state's governor only, and could not be mobilized by the president.

• Organized Reserve Corps, consisting of an Officers' Reserve Corps and an Enlisted Reserve Corps: Essentially a direct reserve for the Regular Army. Organized Reserve units and men practiced similar training routines to the National Guard. A major responsibility of the Regular Army during the interwar period was training the Guard and Reserves; the National Guard also trained officers of the Reserves. The Reserve Officers' Training Corps, authorized by the National Defense Act of 1916, provided voluntary military instruction to students at four-year colleges and universities, who were then commissioned as lieutenants in the Regular Army; institutions acted as "feeder" schools for nearby units. The Enlisted Reserve Corps, as there was no real incentive for men to join it, remained small in comparison. Men were appointed as officers for periods of five years, or served as enlisted men for periods of three years.

• Army of the United States: This "fourth" branch of the Army was established by law on May 14, 1940, but was not formally activated in the guts of the War Department until February 1941. It basically replaced the defunct "Volunteer Army" from the National Defense Act of 1916. During a period of national emergency or war, all men who normally would have been able to voluntarily enlist in the Regular Army, National Guard, or Organized Reserve Corps for the specified enlistment periods would henceforth be enlisted into the "Army of the United States" for a period of the duration of the national emergency or war plus six months. On December 13, 1941, service in the Army of the United States was extended by law to those men who had entered, or would enter in the future, the Army via the Selective Training and Service Act of 1940. Units could also, like in the Regular Army, Organized Reserve Corps, or National Guard, be raised in the Army of the United States, but these units, like the men serving in them, were only "temporary." The ability to enlist in the Regular Army during an emergency or war for three years or the duration of the emergency or war plus six months, whichever was longer, was confirmed by law on June 1, 1945. This law also allowed men discharged from the Army of the United States during an emergency or war to re-enlist in the Regular Army within three months. The portion of Executive Order 9279 which barred voluntary enlistment by men ages 18 to 37 (see below) was revoked by president Harry S. Truman's Executive Order 9605 on August 29, 1945.

During the war, roughly ten million men (60 percent) were drafted into the U.S. armed forces and six million (40 percent) enlisted voluntarily; a maximum of about twelve million were in service at any one time. I talk about the experience of the draftee or voluntary enlistee, more particularly the former, here.

Mobilization and combat is where the designation between Regular Army, Organized Reserve, National Guard, and Army of the United States men and units became heavily blurred. The Regular Army in the interwar period was small and understrength, so large numbers of Organized Reserve officers were called to active duty. Many National Guard officers were removed from their posts and replaced by Regular Army or Reserve officers because they were perceived to be "political" rather than "military" appointees. One Regular Army general officer controversially gave purposefully poor performance evaluations of National Guard officers.

To raise new units in the Regular Army or Army of the United States, cadres were taken from already-active units, with the spaces being filled by newly-inducted draftees or volunteers. Organized Reserve divisions were almost completely "blank slates" upon mobilization. They were authorized, in the interwar period, to have a maximum of all officers and only one-third of key enlisted men. Since there were few enlisted men serving in the Reserves, and many Reserve officers were mobilized individually and sent to other units, their former divisions turned into skeletons, and had to be refilled with cadres from other, nearby, units, or draftees and volunteers.

Amounts of National Guard troops were allotted to states based on population, both in "maintenance" and larger "peacetime" strengths. The latter strength was often not enough to completely fill all units allotted to a state, so the remainders had to be filled with draftees and volunteers. An interesting situation could result in Organized Reserve or National Guard units where draftees or volunteers who never served in the Organized Reserve or National Guard could find themselves with their "local" units; the Army chose to streamline unit activations by sending newly-inducted men from reception centers near where a division was posted for its first training, which was often not far from its assigned location.

This company roster from the 35th Infantry Division, a National Guard infantry division from Kansas, Nebraska, and Missouri, demonstrates the practice. Men with Army Serial Numbers beginning with “2072-” were present with Company L when it was federalized on December 23, 1940. The unit moved to Camp Robinson, Arkansas, in January 1941, and soon reached its full complement with draftees (men whose Army Serial Numbers begin with “37-”) and volunteers (men whose Army Serial Numbers begin with “17-”) from reception centers of the Seventh Corps Area, comprising the states of Missouri, Kansas, Nebraska, Colorado, Iowa, Minnesota, North Dakota, South Dakota, and Wyoming.