What would FDR have done if Hitler hadn’t declared war?

Through Britain FDR would have fought an undeclared air war over Germany similar to the Battle of the Atlantic fought against U-boats. B-17s, 24s and 25s with "volunteer" American airmen under nominal RAF command until their area bombing of German cities eventually provoked Germany to officially declare war, likely within a year. The main question is whether the additional time would have allowed Germany to complete its heavy water experiments and build a nuclear weapon before a differently dated D-Day and before the Soviets reached Berlin without a second front. The late and reduced USAF deployment of targets over German cities would have allowed the Luftwaffe to deploy more resources against the Soviets, slowing the Soviet advance. Before area bombing had much effect on German war production, the primary job of the Anglo-American air war was to provide Stalin with the simulacrum of a second front by putting airborne targets over Germany. My UCLA professor Albert Hoxie worked for the USAF Strategic Bombing Survey in Hamburg shortly after the war. His conclusion: the firebombing of Hamburg helped the German war effort by closing down all the clubs, cabarets, theaters and the red light district. After July 1943 there was nothing to do but work.