Dann, Jack. "Da Vinci Rising"
Divergence: c 1470 CE
What if: Leonardo Da Vinci developed his flying machine with the help of Niccolo Machiavelli.
Summary: The machine is taken over by the Milanese government to use for aerial bombing to conquer the other city-states.
Comments: This story is a revised expansion of text excerpted from Dann's The Memory Cathedral: A Secret History of Leonardo da Vinci, Bantam 1995 (0553096370), Easton 1996, Bantam 1996 (0553378570), HarperCollins Australia/Flamingo 1998 (0732259517). And while this story is alternate history, the novel is instead, as the subtitle indicates, secret history.
Published: In Asimov's Science Fiction, May 1995; Nebula Awards 32: SFWA'S Choice for the Best Science Fiction and Fantasy of the Year (ed. Jack Dann), HBJ 1998 (0151003068BUY, 0156005522BUY); Jubilee, Tor 2003 (076530676XBUY), 2004 (0765306778BUY); and Wildside Double #9, Borgo 2010 (1434411907BUY).