The airplane in which he did it was a Curtiss Model D headless pusher. Headless because it lacked the foreplanes which had previously been used for pitch control and were replaced by trailing elevators. Pusher because the prop was in the rear of the airfoil and "pushed" instead of "pulled" the airplane. Ely's plane used a Curtiss Vee engine, likely of 40 HP, possibly of 75 HP.
Ely made three separate flights from Fort Missoula's parade grounds on that historic day before a crowd of somewhat less than 5,000 onlookers.
The Missoulian published an article on the event the next day, on June 29, 1911 and two days short of 100 years later published a centennial piece on Ely's flights on June 26, 2011. Excerpts from that article are reproduced below.