Some historical background on the US Military in 1940 (thus presumably when one would see any US entry into WWII):
YEAR | ARMY | NAVY | MARINES | COAST GUARD | TOTAL |
1940 | 269,023 | 160,997 | 28,345 | | 458,365 |
1941 | 1,462,315 | 284,427 | 54,359 | | 1,801,101 |
1942 | 3,075,608 | 640,570 | 142,613 | 56,716 | 3,915,507 | - included to give feel for expansion post war entry, though the US entering voluntarily rather than post attack might not witness this sort of rapid expansion |
As far as equipment goes:
The main tank was the M2 light tank (in the M2A2 or M2A3 versions with .50 machine gun,
not the later M2A4 with 37mm gun) along with the similar sized M1 Combat Car:
That said, one could possible see a rapid development of the M2A4 version though I doubt one would see a significant number in service until 1941 at the earliest.
These would be supported by the M2A1 / M3 / M3A1 Scout Car:
And a few 37 mm M3 as a dedicated anti-tank gun (noting this was just entering service in 1940):
On the Howitzer front you had the 75mm Pack Howitzer M1 (though by 1940 there were only 91 in service):
Along with the Canon de 155mm GPF (only 24 in service at the time):
and 155 mm Gun M1 Long Tom just entering service:
When war broke out the US Army Air Corp (it only became the US Army Air Force in mid 1941) had approx 3,305 aircraft in a state of readiness, while the Navy had about 3,000 aircraft ready. For the USAAC though, of its 1,500 combat aircraft, only 800 were rated as first-line. By comparison, the RAF had 1,750 first-line aircraft and the German Luftwaffe 3,750. Of these, the main types were:
B-17: In the real world the USAAC received 39 B-17Bs in 1939–40, 18 B-17Cs in 1940, and 42 B-17Ds in the first quarter of 1941. Even if one therefore pushed the 1941 delivered into 1940 (not a definite thing because there would be a need for factory capacity), one might see some 57 - 100 odd available in 1940;
B-18 Bolo: Some 350 in service;
A-17: Some 400 in service;
DC-2/3 (as C-32/C-39): Approx 24 in service;
P-35: Some 100 odd in service;
P-36: The main fighter in terms of numbers with some 200 in service;
P-38: Only just entering service;
P-39: An initial order for 80 aircraft had been placed in only August 1939 so would have just been entering service; and
P-40: Only just entering service.
The USN was probably the best equipped though as to what this may have offered in 1940 for a largely European continental war is debatable. Leaving out the Battleships therefore, if one considers the carriers, then really just the following available in 1940:
USS Lexington (CV-2)
USS Saratoga (CV-3)
USS Ranger (CV-4)
Yorktown (CV-5)
Enterprise (CV-6)
USS Wasp (CV-7): Just commissioned and probably able to be rushed into service.
These would have carried a mix of the following:
Grumman F2F: Would have been exiting service but still an option if needed;
Grumman F3F: The main fighter in service but only with about 140 odd in service at the time;
Brewster F2A Buffalo: Probably the best (potential) type available at the time with some 11 F2A-1s and 43 F2A-2s available to the USN/USMC;
Grumman F4F-3: Maybe available but only if rushed into service;
Vought SB2U Vindicator: maybe some 150 in service; and
Douglas TBD Devastator: some 130 in service.
One has to remember though that the USN's main focus was the Pacific with Japan so would have been unwilling to commit more than about 2 - 3 of the carriers to any European War.
Generally, I would argue that any early entry to WWII by the US would only really see them using what was already in service in 1940 and in limited numbers at that. Therefore, they would not have made a great contribution and in fact, may have even wanted to utilise some of the European equipment. Moreover, without some sort of Casus belli I can't see a significant ramp up in either production/recruitment /introduction of new types that was later generated by the Pearl Harbour attack.