This is just one web site of many that explains the deal. You are welcome to do your own research. Just search " american corporations that supplied germany in ww2" or something similar.
Some of these corporations had their own manufacturing facilities and some were just collaborators.
http://www.threeworldwars.com/world-war-2/ww2-background.htm
American Business in the War
Not only was the government concerning itself with a possible war with Japan, but it was also aware that American capitalists were creating a war machine in Germany in the early 1930's, years before Germany started their involvement in World War II.
William Dodd, the U.S. Ambassador in Germany, wrote Roosevelt from Berlin:
At the present moment, more than a hundred American corporations have subsidiaries here or cooperative understandings.
The DuPonts have their allies in Germany that are aiding in the armament business. Their chief ally is the I.G. Farben Company, a part of the government which gives 200,000 marks a year to one propaganda organization operating on American opinion.
Standard Oil Company... sent $2,000,000 here in December, 1933 and has made $500,000 a year helping Germans make ersatz [a substitute] gas [the hydrogenation process of converting coal to gasoline] for war purposes; but Standard Oil cannot take any of its earnings out of the country except in goods.
The International Harvester Company president told me their business here rose 33% year [arms manufacture, I believe], but they could take nothing out.
Even our airplanes people have secret arrangements with Krupps.
General Motors Company and Ford do enormous business here through their subsidiaries and take no profits out.16
In addition to these American companies, others were assisting the Germans in creating the materials they needed to wage war. For instance, International Telephone and Telegraph (I.T.T.) purchased a substantial interest in Focke-WoIfe, an airplane manufacturer which meant that I.T.T. was producing German fighter aircraft used to kill Americans.
I.G. Farben's assets in America were controlled by a holding company called American I.G. Farben. The following individuals, among others, were members of the Board of Directors of this corporation:
Edsel Ford, President of the Ford Motor Co.;
Charles E. Mitchell, President of Rockefeller's National City Bank of New York;
Walter Teagle, President of Standard Oil of New York;
Paul Warburg, Chairman of the Federal Reserve, and the brother of Max Warburg, the financier of Germany's war effort, and
Herman Metz, a director of the Bank of Manhattan, controlled by the Warburgs.
It is an interesting and revealing fact of history that three other members of the Board of Governors of the American I.G. were tried and convicted as German "war criminals" for their crimes "against humanity," during World War II, while serving on the Board of Governors of I.G. Farben. None of the Americans who sat on the same board with those convicted were ever tried as "war criminals" even though they participated in the same decisions as the Germans. It appears that it is important whether your nation wins or loses the war as to whether or not you are tried as a "war criminal."
It was in 1939, during the year that Germany started the war with its invasions of Austria and Poland, that Standard Oil of New Jersey loaned I.G. Farben $20 million of high-grade aviation gasoline.
The two largest German tank manufacturers were Opel, a wholly owned subsidiary of General Motors and controlled by the J.P. Morgan firm, and the Ford subsidiary of the Ford Motor Company.
In addition, Alcoa and Dow Chemical transferred technology to the Germans, as did Bendix Aviation, in which the J.P. Morgan-controlled General Motors had a major stock interest, which supplied data on automatic pilots, aircraft instruments and aircraft and diesel engine starters.
In addition to direct material support, other "capitalistic" companies supplied support: In 1939 the German electrical equipment industry was concentrated into a few major corporations linked in an international cartel and by stock ownership to two major U.S. corporations (International General Electric and International Telephone and Telegraph.)
Further support for the American owned or controlled corporations came during the war itself, when their industrial complexes, their buildings and related structures, were not subject to Allied bombing raids: "This industrial complex (International General Electric and International Telephone and Telegraph) was never a prime target for bombing in World War II. The electrical equipment plants bombed as targets were not affiliated with U.S. firms."17
Another example of a German General Electric plant not bombed was the plant at Koppelsdorf, Germany, producing radar sets and bombing antennae. Perhaps the reason certain plants were bombed and others weren't lies in the fact that, under the U.S. Constitution, the President is the Commander-in-Chief of all armed forces, and therefore the determiner of what targets are bombed.
The significance of America's material support to the German government's war efforts comes when the question as to what the probable outcome of Germany's efforts would be: "... not only was an influential sector of American business aware of the nature of Nazism, but for its own purposes aided Nazism wherever possible (and profitable) with full knowledge that the probable outcome would be war involving Europe and the United States."18
Even Hitler's ideas about exterminating the Jews were known to any observer who cared to do a little research. Hitler himself had written: "I have the right to exterminate millions of individuals of inferior races, which multiply like vermin."
In addition, Hitler made his desires known as early as 1923 when he detailed his plans for the Jews in his book Mein Kampf. Even the SS Newspaper, the Black Corps called for: "The extermination with fire and sword, the actual and final end of Jewry."
This material support continued even after the war officially started. For instance, even after Germany invaded Austria in March, 1938, the Ethyl Gasoline Corporation, fifty percent owned by General Motors and fifty percent by Standard Oil, was asked by I.G. Farben to build tetra-ethyl plants in Germany, with the full support of the U.S. Department of War which expressed no objection to the transactions.