Although the Works Progress Administration (later renamed Work Projects Administration) is not usually thought of as a defense-related agency, it did have a role in getting the United States ready to participate in World War II. It did this mainly by building military facilities and other infrastructure.
The original aim of the WPA was to provide work for people who were unemployed due to the effects of the Great Depression. The WPA sought to do this mainly by employing large numbers of people for public works projects. These involved things like building bridges, roads, and public buildings. The WPA also famously employed people like writers and artists to create things like oral histories of slavery and to teach art classes around the country. By employing these people, the WPA helped to reduce the impact of the Depression.
We can say that the WPA helped prepare the US for WWII in a number of ways. First, and very indirectly, it helped by giving people jobs in the years before the war began. This would have improved the mental and physical health of many men and made them more fit for service. It would have improved the morale of the country as well. Second, the WPA improved the country’s overall level of infrastructure. It built bridges and roads, improving transportation. It extended electrical grids, making it easier to place factories in various places around the country. By doing these things, it made it easier for the US to engage in the sort of massive industrial effort needed to fight in WWII. Finally, and most directly, the WPA was engaged directly in building things that were important to the war effort. This was particularly true in the later years of the WPA’s existence (it was dissolved in 1943). The WPA was involved in building many military installations. It built things such as military bases and armories. By doing so, it helped the US be ready to mobilize the huge military force that fought and won the war.