Germany did pretty much steamroll Western Europe up to the coast, except for Spain and Portugal. Taking Britain was going to be a lot harder, and everybody knew that. The German plans to invade southern England in 1940 are positively ludicrous, and their attempts to suppress British air power caused them losses they never really had a chance to recoup.
Germany then attacked the Soviet Union, and failed in 1941, before the US had any significant impact on the war. The US didn't have an army on the European mainland before September 1943, and the US air forces didn't have a serious impact before mid-1943. Eventually, the US had a major role, but it came much later than any time Germany could possibly have won, and primarily accelerated the end of the war and kept the Soviets out of most of Western Europe.
The US was engaged in the Pacific, and the majority of the resources available in 1942 went there, but the US was really not ready for a war by the end of 1942. The US was frantically building up a large army, but it didn't start to show up until sometime in 1943 and wasn't anywhere near full strength before 1944.