Anyways, surely the history would be very different had the US deployed earlier!

Very true, it could have resulted in a lot more dead US, British, and Allied soldiers for little additional gain!

Simply put the US military was in no way shape or form prepared for an invasion of Northwest Europe even by late 1942. It had too few troops and units trained up, insufficient numbers of every piece of equipment, from bazooka's, to howitzers, to tanks, to aircraft of all types. Especially in the face of a German military which if not as deeply entrenched was still far less bled dry.

While the force was as a whole extremely untested, both in combat experience of leaders at all levels, but in other important areas such as doctrine and unit organization. To make the invasion of France the first test with an army of draftee's inducted for at most maybe 18 months by and large is courting disaster especially as supporting arms such as the Army Air Corps are undergoing similar expansions.

That is not to say that plans werent drafted and didnt have some support. Specifically there were SLEDGEHAMMER and ROUNDUP, which would have envisioned landings in Fall of 1942 or Spring of 1943 respectively. SLEDGEHAMMER would have been a very shoestring operation as well, to be launched with a force comprised of more British troops than US troops as divisions were still being formed and sent overseas, and was contemplated only if events such as the impending collapse of Soviet forces made it required or if US domestic concerns came to a head about the lack of visible efforts. ROUNDUP would have been similar but on a slightly larger scale, but again a heavily British force, and the beachheads would have net been expected to see immediate breakouts but an extensive period of buildup before a new offensive.

Frankly the British were shocked and horrified at the proposals, especially as they were going to be putting up many of the troops for it. Besides the lack of supporting forces, amphibious equipment and experience, and US inexperience, they desired efforts elsewhere. Specifically in North Africa where the Mediterranean could be made safe for Allied shipping, and the Italians taken out of the war ideally by attacking Europe's soft underbelly as it were, but first the Afrika Korps and General Rommel and friends had to be dealt with.

In a series of conferences in the first months of 1942 a united front of British senior leadership managed to win over FDR and gain support for TORCH, which would land a mostly US, with UK support, force in French held North Africa. Then this force and the British 8th Army out of Egypt they would squeeze the Italian and German forces in Africa between them back to Tunisia.

So the landings under TORCH go forward in November 1942 and the fighting lasts into early 1943. Following that is HUSKY in July of 1943, and Italy itself in September, so in reality there were US boots on the ground from the summer of 1943 on in Europe proper. Then the Allied command sets the date for OVERLORD and the invasion of France for mid 1944 as the buildup of forces in the UK continued and planning goes forward, some units and leaders which had now gained experience in Africa, Sicily, and Italy are also pulled back to Britain for rest, refit, and to be used in OVERLORD.

In late 1942 the US military was simply not ready to support the sort of power projection required to win or even hold a large scale force in NW Europe. It should be noted that the two major amphibious landings that were conducted in the second half of 1942 were full of mistakes, close calls, bungling, and important lessons to be learned. WATCHTOWER in the Pacific on Guadalcanal and Tulagi was pulled of in a rush and sustained at times by the narrowest of margins, while TORCH was aided by halfhearted and quickly crumbling Vichy resistance and the lack of better prepared German or Italian defenders and even then was touch and go in places.

But in general this idea that the US Army at least was sitting idle before Normandy in Europe is very much incorrect, many units were being sent to train in Britain explicitly in preparation for Normandy, but many others were in combat with German forces within a year of Pearl Harbor. Just wisely in a series of campaigns meant to seize less well defended Axis territory to aid Allied plans and season the US military and ease into the practicalities of the US-UK joint command relationships.