Why an early R-4360 would influence 1941 aircraft, not 1939 fleets

What changes if the Pratt & Whitney R-4360 is available in 1939?

Aircraft already in service do not change, but American designs entering service from about 1941 could become more capable.

An engine does not create a combat aircraft immediately because designers need years to build an airframe around its dimensions, cooling, fuel use, and power. Making the R-4360 available in 1939 arrives too late to alter aircraft already in production or British strength at the outbreak of war. A roughly three-year design cycle places the first meaningful effects around 1941. The counterfactual therefore improves later American wartime aircraft rather than transforming the opening campaigns.