Very much so. (Rolls Royce were in fact more an aircraft engine manufacturer than a car maker for most of their history).

Military manufacturing in the US was originally a virtual cottage industry with many war machines being made in arsenals and under supervision of the military services. The civilian market was always larger and more attractive.

Desperately mobilising in the late 1930s, the British and French arrived in the US looking to make arms deals, with cash in hand. Already Ford had European branches involved in war manufacture. However, even in this situation US manufacturers were reluctant to undergo all the tooling and work necessary, given the precarious future France and Britain seemed to be facing.

Roosevelt encouraged, cajoled and virtually forced US car makers to gear up for war, realsing that the British money would kick-start the building of an industrial base that the US would need once the war was joined. The system was to provide contracts and funds, and to use the big US manufacturers to actually set up and operate the production lines (and make the profits).

So Chrysler ran the largest tank plant in the Western world and built tanks used by almost all the Allies. Ford built tanks of the same design, with their own engines. Cadillac as well. And various companies that had made railway carriages.

But the changes went beyond Detroit and converting factories. A centrally-planned system saw designs from one company manufactured by another. The Consolidated Aircraft company was asked to build Boeing's B-17 heavy bomber. They offered to make a superior design with no delay in getting it into production, the B-24. Ford built and operated a plant at Willow Run, which pumped these aircraft out, and it became the most-produced US aircraft of the war. Which is amazing because whereas other nations' most produced aircraft were single engined fighters of light attack aircraft, the US's was a four-engined heavy bomber. And it was one of three such aircraft produced in large numbers. In addition to tens of thousands of fighters and other planes.

Unlike other countries, where a single manufacturer would usually design and build an aircraft, with a second or third adding their production facilities for very successful designs, in the US the government would fund and assign these mass production experts to build a huge factory from nothing, again and again. The thing being produced was merely what would in today's jargon be a 'widget', be it a P-51 Mustang or a Rolls Royce Merlin engine or an aircraft carrier- the production facility would be made to fit the task as perfectly as possible.

In this way, with massive economies of scale and unlimited access to capital, raw materials, labour and markets, and with massive government assistance in R and D and organisation, US manufacturing became super-efficient in a way that eluded other nations.

Multiple major US companies actually had german subsidiaries that manufactured arms, or helped the Nazis. Ford had a factory in Cologne staffed by a thousand Russian POW's. Hitler was very fond of Henry Ford having his portrait in one of his offices.

http://rarehistoricalphotos.com/henry-ford-grand-cross-1938/

General Motors was sued by Holocaust survivors for the machines of war that they helped build. IBM of course developed the system the Nazis used for cataloguing Jews and others in their death or work camp system.

Chase Manhattan Bank in many ways financed the Nazis. Germany had significant help from many large US corporations.

I think some US companies actually got to sue the US government for bombing their factories in Germany and won damages!