WWII is viewed as the quintessential example of fiscal stimulus and exerts an outsized influence on fiscal multiplier estimates, but the wartime economy was highly unusual. I use newly-digitized contract data to construct a state-level panel on U.S. spending in WWII. I estimate a relative fiscal multiplier of 0.25, implying an aggregate multiplier of roughly 0.3. Conversion from civilian manufacturing to war production reduced the initial shock to economic activity because war production directly displaced civilian manufacturing. Saving and taxes account for 75% of the income generated by war spending, implying that the add-on effects from increased consumption were minimal.
That is from a 2018 paper by Gillian Brunet, and you will note that it reflects the consensus of the literature as a whole. I do favor the federal government borrowing and spending a great deal of money right now on things that we need. If you think we are in a traditional Keynesian scenario, or are pulling out a traditional AS-AD model, you are going to be very badly disappointed. Most of all, we need to be spending more on public health and remedies for Covid-19. Here is my earlier Bloomberg column on analogies and disanalogies between Covid-19 and World War II. And again, see Garett Jones and Dan Rothschild on the 2009 stimulus.