To do this you need to seriously tone down if not end the Cold War before it begins. So have Stalin trip and break his neck somehow near the end of the war and someone more "moderate" come out on top of the USSR. Meanwhile, have Henry Wallace remain FDR's VP in '44 and become president after he dies. Without the acrimonious East/West relationship you'd have room for the UN to take a more active role in things like regulating international trade, developing its own independent military, and potentially even commanding a nuclear arsenal as some folks wanted it to in '45. There's limits to how much sovereignty nations will surrender to the UN, but if you can build a stable international consensus out of the postwar ashes and have it be successful you might be able to turn it into a sort of proto-EU.