I don’t believe any new musical instruments originated in the USA (I am no expert, however.) But it all depends on how you define “new.” There have been many modifications of existing instruments that originated here: electric solidbody guitar, 5 string banjo, resonator guitar, synthesizers, electric pianos, etc. But they were all modifications or advancements of existing instruments. I guess you could argue for washboard or jug (which were not designed as a musical instrument) or some other homemade instruments, but nothing in common use.

Of course it is semantics. Is a solidbody electric guitar a new instrument, or simply a variation of a hollowbody Spanish guitar?

Is a Rhodes electric piano or a Moog synthesizer a new instrument or a variation of a piano? You could argue for both. I might fall on the side on new instruments as neither uses strings like a piano.

Native Americans played flutes and drums, as have all humans throughout history, but it would be hard to determine their origin.

Still, I have to pay my respects to Les Paul for pioneering the solidbody electric guitar and multitrack recording. The Wizard of Waukesha changed music forever and for me in particular as an electric guitar player. I got to see him in concert and meet him at the Iridium Jazz Club in New York. He was age 93 at the time.