Only militarily did the US become a superpower in WW2.
I was an economic powerhouse long before then.
The 1930's US stock market crash brought down the entire world!
As another point, it's not possible to make a ton of money selling weapons if you don't already have a thriving manufacturing sector. You have to at least be doing well to begin with.
According to [0], the US reached parity with the UK (probably the richest country in Europe at the time?) in terms of GDP per capita in 1880!
[0] https://www.rug.nl/ggdc/historicaldevelopment/maddison/relea...