thinking taking a look at stride would be worthwhile. It's considered one of the first truely original American music styles for the piano ( rag was mentioned as was 'jazz' in the Gerswin reply) but stride isn't quite rag and it's not quite what we think of as jazz, sort of falls between ,
some notable examples
James Price Johnson Carolina Shout (1918/1921), Mule Walk (1939), Caprice Rag
Thomas "Fats" Waller Handful of Keys (1929), Vipers Drag (1934), Alligator Crawl (1934)
Willie "The Lion" Smith Finger Buster (1931), Echoes Of Spring (1939)
also Macdowell's Op. 62 New England Idylls (1902) set of pieces for piano comes to mind as 'american'.
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fyi
Like its forebear, ragtime piano, stride piano is highly rhythmic and somewhat percussive in nature because of the "oom-pah" (alternating bass note / chord) action of the left hand. In the left hard, the pianist usually plays a single bass note, or a bass octave or tenth, followed by a chord, while the right hand plays syncopated melody lines with characteristically blues-based embellishments and fill patterns.
James P. Johnson, known as the "Father of Stride", created this unique style of jazz piano along with fellow pianists Willie "The Lion" Smith, Fats Waller and Luckey Roberts. Johnson's greatest contribution was to recast the "straight" feeling of ragtime with a more modern, swinging beat.[1] He discovered and employed the tenth or "broken tenth" interval to introduce more swing in his left hand. This can be heard in his famous composition "Carolina Shout". The pianist could not only substitute tenths for single bass notes, but could also play broken (staggered) tenths up and down the keyboard[2] in scale fashion—an innovation that subsequently inspired boogie-woogie and the eventual transition to modern four-beat jazz.