Moving away from the world cup scene, its worth mentioning the Descenders from the 80s as downhill specific machines. OK they were built in very limited numbers but you could hardly call them a XC bike with a lowered saddle.
As Elev12k has pointed out the Verlicchi is really the first good example of a production DH specific bike, and as the tracks became more technical and moved away from simple dirt tracks the bikes and geometry changed to suit, and you start getting bikes like the Turner Burner DH from around 95 onwards with no provision for a front derailleur.
For me the start of the DH specific thing came with people fitting larger big chain rings and chain devices and sacrificing there performance in the XC and uphill race in order to concentrate on the DH - over here the uphill always preceeded the DH, how else did you get to the start...
and there used to be an overall winner as well as the different categories (XC, UH, DH)