I think the Flamingo was the first thing on the site.

According to the LV Strip History website:

The original owner of the 40-acre tract was Charles "Pops" Squires, who paid $8.75 an acre. In 1944 he sold it to Margaret Folsom for $7,500. In 1945 Billy Wilkerson bought the land, hired an architect to design the luxury hotel/casino, began construction, and ran out of money. Enter...Bugsy Siegel.

No mention of anything else on the property.

http://www.lvstriphistory.com/ie/flam.htm