Immediately before and during WWII, the US Army had four parts

Regular Army

National Guard

Organized Reserves

Army of the United States

When a man joined the Army voluntarily, he would be sent to a reception center where an interview would be conducted and a questionnaire given; the man would list his job in civilian life, and whether he preferred a certain branch, such as the Air Corps or Armored Force; only 5 percent of those men who opted to "pick” their job chose the Infantry or the Armored Force. The realities of mass enlistment sometimes got in the way of this job-picking, and men were often assigned wherever there was room, based upon Tables of Organization and Equipment and manpower of active units. For the filling of these positions, the Adjutant General’s Office computed a formula that converted the needs of every unit into a rate per 1,000 men; the formula might say that out of a certain group of 1,000 inductees, 177.5 were to be truck drivers, 32 were to be riflemen, and 19, surgical technicians. The man was then assigned a Specification Serial Number (SSN) that attempted to correspond to his job in civilian life; SSNs below 500 (like 345, Driver, Truck, Light) had some equivalent in the civilian world; SSNs above 500 (like 745, Rifleman) did not. Draftees who were received by the Army were classified based upon the results of their mental and physical examination, as well as the Adjutant General's formula.

In the entire Army, 778 out of every 1,000 men were assigned civilian-type jobs. If broken down by specific branch, like Infantry, only 164 out of every 1,000 Infantry branch troops had a civilian-type job. A problem with this method of assignment was that men who had held skilled trades ended up being assigned to the "higher-intelligence" non-combat elements of the Army, or the Army Air Forces, leaving the Infantry with men of subpar morale or motivation. Very intelligent men went to the Army Specialized Training Program, which was broken up in early 1944 and its personnel transferred to infantry divisions as it was not producing enough officers. To ease the intelligence problem, the War Department stipulated that from reception centers, each branch would receive an equal proportion of higher-intelligence personnel, or those men capable of becoming officers.

Voluntary enlistment into the United States military was eliminated on December 5, 1942 via executive order to prevent unnecessary disruption of war industry. At the time, the draft age was 18-37; eligible men who were 17, and those who were 38-45 were not subject to the draft, but could still voluntarily enlist provided they met the entrance requirements. See the section on the Army of the United States below; I provide more information on the status of draftees.

Regular Army

The numerical designations 1-25 were nominally reserved for Regular Army divisions. These divisions were active before US entry into World War II.

• 1st-9th, 24th, and Philippine (25th) Infantry Divisions

• 1st-5th Armored Divisions

• 82nd and 101st Infantry (Airborne) Divisions

• 1st and 2nd Cavalry Divisions

National Guard

The numerical designations 26-45 were reserved for the National Guard divisions. There were several exceptions; the 42nd Infantry Division was classifed as a Guard division, but was made up of draftees from all over the USA; the 43rd Infantry Division was an Army of the United States division. The 39th Infantry Division, of National Guard troops from Louisiana and Arkansas, was not activated until after WWII. Men who joined the National Guard prior to WWII were placed into divisions based upon their state of residence. Selected towns or cities would have an armory, with one or multiple division sub-units consisting of men drawn from said town or city and the surrounding area reporting to it. Men trained for one weekend per month and one week per year. As casualties mounted during WWII, drafted replacements from all over the country were used to fill spots in unit rosters; as a result of this, most of the casualties from National Guard divisions actually came from outside their recruiting areas. As an example, the 35th Infantry Division consisted of the 134th Infantry Regiment from Nebraska, the 137th Infantry Regiment from Kansas, and the 320th Infantry Regiment made up of draftees and meant to fill out the division.

134th Infantry Regiment Stations

Unit	Town/City
Regimental HQ	Omaha
Antitank Company	Gering
Cannon Company	Unknown (not in publication)
Service Company	York
Medical Detachment	Omaha
1st Battalion HQ	Nebraska City
Company A	Nebraska City
Company B	Falls City
Company C	Beatrice
Company D	North Platte
2nd Battalion HQ	Omaha
Company E	Scottsbluff
Company F	Hastings
Company G	Hartington
Company H	Grand Island
3rd Battalion HQ	Lincoln
Company I	Lincoln
Company K	Omaha
Company L	Omaha
Company M	Seward

National Guard Divisions

Division	Area
26th	CT, ME, MA, NH, RI, VT
27th	NY
28th	PA
29th	VA, MD, PA, Wash., D.C.
30th	NC, SC, GA, TN
31st	AL, FL, LA, MS
32nd	MI, WI
33rd	IL
34th	ND, SD, IA, MN
35th	KS, MO, NE
36th	TX
37th	OH
38th	IN, KY, WV
40th	CA, UT
41st	ID, MT, OR, WA
42nd	Draftees
44th	NJ, NY
45th	AZ, CO, NM, OK

Organized Reserves

The range 76-106 was nominally reserved for Organized Reserve divisions, although there were several exceptions; a couple Army of the United States divisions lie within it.

Most of these divisions were newly activated during and then fought in World War I. They were then deactivated afterwards. In the 1930s, the United States created an Organized Reserve of 32 divisions. Instead of creating more new divisions, the deactivated WWI-era divisions were revived in the Reserves. A pool of trained reserve officers and men was assembled who could be used to quickly activate the divisions again.

During World War I, these divisions were part of the National Army (the World War I-era draftee force) and assigned draftees from certain states. This process was not used during World War II. An interesting result of the WWI-era process is that the new divisions sometimes created their insignia in this way; the 91st “Wild West” Infantry Division’s unit insignia has a pine tree, as many men were from Oregon and Washington. The 80th Infantry Division gained its nickname “Blue Ridge” as many draftees were from the Blue Ridge Mountains area.

During WWII, the Organized Reserve divisions, after having their reserve officers and men reoriented, were filled up with enlistees and draftees.

The Organized Reserve divisions that served in WWII were the;

• 76th-81st Infantry Divisions

• 83rd-91st Infantry Divisions

• 94th-97th, 99th and 100th Infantry Divisions

• 102nd and 103rd Infantry Divisions

WWII-era Organized Reserve Division	World War I-era draftee area
76th	ME, NH, VT, RI, CT
77th	NY
78th	NY, NJ, DE
79th	PA, MD, Wash., D.C.
80th	NJ, MD, VA, WV, PA
81st	NC, SC, TN
83rd	OH, WV
84th	IN, KY
85th	MI, WI
86th	IL
87th	AR, LA, MS
88th	ND, SD, MN, NE, IA, IL
89th	KS, MO, CO
90th	TX, OK
91st	CA, OR, WA, UT, ID, NV, MT, AK
94th	Organized post-WWI
95th	Organized post-WWI
96th	Organized post-WWI
97th	OK, MN; no combat in WWI
99th	Organized post-WWI
100th	Organized post-WWI
102nd	Organized post-WWI
103rd	Organized post-WWI

Army of the United States

The Army of the United States (AUS) was the Army's conscripted force during WWII. These divisions, with the exception of the 42nd, 43rd, 92nd, and 93rd Infantry Divisions, were activated completely from scratch, having no previous lineage.

These divisions were the;

• 10th Mountain Division

• 23rd, 25th, 43rd, 63rd, 65th, 66th, 69th-71st, 75th, 92nd, 93rd, and 106th Infantry Divisions

• 6th-14th, 16th, and 20th Armored Divisions

• 11th, 13th, and 17th Airborne Divisions

Divisions in the Army of the United States were activated by;

• Drawing a small officer and enlisted cadre from an already-active division

• Assembling and training the division's junior officers

• Filling it with enlistees, draftees, and transfers

• Commencing training

During the period immediately after US entry into World War II, the divisions of the Army of the United States were for the most part not activated yet and don’t really have a bearing on your specific question. I talk more about the process of activation of a typical AUS division and the status of draftees here

On draftees, there was the problem of 18 year olds. During WWII, half of all draftees were 18 years old, a demographic consequence of the draft. In February 1944, the War Department directed that 18 year olds with less than six months of training were not to be shipped as replacements unless no other men from any other source could be found. In June 1944, an absolute ban was imposed; no man younger than nineteen would be shipped overseas as an infantry or armored replacement, and no man younger than eighteen years and six months would be assigned to an infantry or armored replacement center. To comply with the restriction, it was necessary to "store" many of these men after their graduation by assigning them to divisions still in the United States not expected to deploy for a while; fourteen infantry and three armored divisions. The ban was found to be impractical, "holding back" over 22,000 men that the Army needed in the United States, and was rescinded in August 1944. 18 year olds began to be shipped as replacements again in December 1944.

Sources:

134th Infantry Regiment History

US World War I Order of Battle

The Procurement and Training of Ground Combat Troops, by Robert R. Palmer, Bell I. Wiley, and William R. Keast