Was Smedley Butler right when he said 21,000 million/billionaires were created from World War I?

Question

In his book entitled "War is a Racket" Smedley Butler argues that the profit of war is counted in bodies and money spent. He also wrote that 21,000 millionaires/billionaires were made from World War I. Is this true? If it is true, how did these people become so wealthy?

"In the World War [I] a mere handful garnered the profits of the conflict. At least 21,000 new millionaires and billionaires were made in the United States during the World War. That many admitted their huge blood gains in their income tax returns. How many other war millionaires falsified their tax returns no one knows."

-quote from an online PDF copy of the book "War is a Racket" by Smedley Butler

Answer 1

This publication from 1920 seems to have taken a look at the millionaire claim:

There was definitely a boost in the economy, as war requires and consumes tremendous material production, which can be seen looking at the USGDP numbers for this period. Note the stagnant years before the war, but production increased steadily after:

1910    33.4
1911    34.3
1912    37.4
1913    39.1
1914    36.5
1915    38.7
1916    49.6
1917    59.7
1918    75.8
1919    78.3
1920    88.4

...and 1916 did see the worlds first billionaire in John D Rockefeller, but the 20,000 millionaire claim may be a manipulation or misinterpretation of statistics.