The only military weapons that were invented in the 1920's or before which is still around today is the Ma Deuce and the 1911, courtesy of John Browning.
Sure, the 1911 might not be issued to regular forces here, but the action has been copied and in slightly modified form, is the basis for most of the world's pistols today.
There are those who have tried to replace the M2HB, but nothing really has taken there.
If you want to go way, way back, take Dr. Gatling's invention and spin it with an electric motor and call it a Vulcan.
What we haven't invented here, we have improved on greatly with our own R&D, from the Harrier to the nuclear aircraft carrier, the M-1 Abrams, submarines and MLRS systems.
We didn't invent it all, but I'd say we have the most toys on the block.