Now there are 300M Americans
As explained in this article, according to census projections, the American population today passed the 300 million mark. One American is born every 7 seconds; one American dies every 13 seconds; one migrant arrives in the USA every 31 seconds.
It has taken the USA 39 years to add the last 100M population and it is expected that it will take 37 years to add another 100M. So, in 2043, there will be 400M Americans. Growth is projected to plateau around 2070.