The job of the US isn’t to win the game, it’s to STOP the Axis from winning the game and make sure the Allies don’t LOSE the game.
As long as the USA can keep the game going, in terms of turns, -usually-
You need to spend on both fronts, as required, with intelligent and mostly unscripted builds. It depends on the skill level of your allies, your overall comprehensive strategy, and the results of opening axis manuevers and attacks.
That said it’s also not what you build, but how you use what you already have. EVERY starting unit, by US 3, if at all possible, should be off of thier islands and the continent, and heading towards theatres where they can be effective. In a central and organized manner, including aa guns. IE Don’t build infantry, or transports, just to leave them sitting around.