A war doesn't directly improve the economy. It's spending on industry that was justified by the war (plus national fervor) that helped with this. But building ships and planes and tanks is not fundamentally useful for civilian industry. If the same level of investment had been directed to civilian industries instead of military, the economy would probably have been even higher.

I do agree that the relative value of the US economy shot up directly due to the destruction the war brought to European, Chinese, Japanese, and USSR industry and populations - so if you were discussing things in this sense, I can agree.