For the most part, the United States was self-sufficient in the most basic raw materials for the war industry. petroleum, coal, iron ore, aluminum, and hydroelectric power were available from domestic sources.

One notable exception to U.S. self sufficiency was rubber. Rubber was in pre-war times largely sourced worldwide from Southeast Asia. Japan being much closer to S.E. Asia, it was realized in the late 30s and 40s that a war could imperil access to rubber. Accordingly, a process for producing synthetic rubber was created along with a huge crash program to ramp up synthetic rubber production. Gasoline rationing in the U.S. was largely instituted to reduce driving and conserve rubber tires.

However, despite being mostly self sufficient, the U.S. also had access to international trade networks in the western hemisphere and parts of Africa. The British empire and commonwealth also traded with the U.S. thru a process called reverse lend lease, sending raw and manufactured goods to the U.S. in partial exchange for the more substantial amount of military goods shipped to the British. Similarly, China shipped Tungsten and Tin back to the U.S. in exchange for it's lend lease products. Lastly, the U.S.S.R. also provided some trace metals like Manganese and Chrome back to the U.S. in exchange for lend lease supplies. I don't have precise figures for total imports to the U.S. but generally having access to world trade was very helpful to the war effort of the U.S. and Britain. This access is the reason that the U.S. did not suffer from shortages of useful trace materials like tungsten and manganese. Additionally, having access to the world economy helped avoid production bottlenecks and expensive work-arounds.

So the U.S. did not "have" to import anything. However they did receive a lot of imports from countries outside Axis control during the war, and this was one of the factors that enabled the U.S. and Britain to outproduce the Axis powers. I don't have any quantitative analysis of this effect immediately available, perhaps another commentator would be able to dive into this even more deeply.