Since this is an alternate history board, I decided to post a genuine aviation WI grounded in reality :
Do you remember Percy Pilcher, the Scottish aviation pioneer from the late 19. century ? He's famous mainly because of his advanced hang gliders, which he built at the time (another pioneer in this field was, of course, Otto Lilenthal).
http://en.wikipedia.org/wiki/Percy_Pilcher
Well, seems he also developed a fully functional steam-powered triplane in 1899 :
http://en.wikipedia.org/wiki/Percy_Pilcher#Renewal_of_interest
http://www.bbc.co.uk/science/horizon/2003/percypilcher.shtml
He never flew on it, though, since he died the very day, on which he was supposed to test it. About 6 years ago, the crew from the BBC TV programme
Horizon built a full-scale exact replica of the craft. The damn thing took off and had flown at least the same distance the Wright Flyer did on it's first flight in late 1903 !
I just wonder... If Pilcher was succesful... Would this really affect aviation history ? Would there be a "head start" of a few years compared to OTL aviation ? Would many early planes be powered by steam engines instead of diesel ones ? (Mind you, Pilcher's plane was a steamer...)