Alex Brandon/AP

President Trump has invoked the Defense Production Act for the first time, requiring General Motors to supply ventilators.

"Today, I signed a Presidential Memorandum directing the Secretary of Health and Human Services to use any and all authority available under the Defense Production Act to require General Motors to accept, perform, and prioritize federal contracts for ventilators," Trump said in a statement.

Some context: US automakers have come to the rescue when the nation has faced supply shortages during wartime in the past. Ford built heavy bomber airplanes and GM built amphibious assault craft, among other things. So it seems only natural that, in the rush to address the critical shortage of ventilators in the US due to the coronavirus pandemic, automakers would again be among the first to answer the call to help.

Last week, Trump tweeted that automakers have the green light to make ventilators, although he stopped short of issuing formal orders to do so under the Defense Production Act at that time.

Nevertheless, Ford, GM, Toyota and Tesla, which have all temporarily shut down their factories in recent weeks, have pledged to help.

But switching from cars to ventilators is not so easy. Ventilators are complex machines that use sophisticated software and specialized parts, and companies that seek to manufacture them face several hurdles — including intellectual property rights, the need for specially trained workers, regulatory approvals and safety considerations.