To add my two pence worth to this thread, my experience finds that Chaparral were the first to try to achieve downforce by profiling the underside of a race car. For those lucky enough to have a copy of "Chevrolet=Racing, 14 Years of Raucous Silence" (first edition,printed 1972), if you turn to page 103 you will see a side view of the Chaparral 1 (1963 car) showing a curved underside profile. Reading the text below the photo states that "the theory of the moment" was to intentionally curve the underside in order to create half a venturi in order to create a low pressure area beneath the car, holding it down. Unfortunately, all they achieved was lift ! As is the case with many concepts, one or two vital elements were missing that would have made the concept work. In this case it was the addition of skirts, which is what Colin Chapman and his designers did with the Type 78 and more effectively with the 79.
So, is it Chaparral, McLaren and finally and the most successful, Lotus ? Or, is it really McLaren first because they did achieve a degree of downforce. Anyway, maybe someone can up with an earlier example ?
Well, maybe one pence at a pinch, but that's being generous. Jim Hall in his racing days was a brilliant engineer, I'd give him full credit for all kinds of clever ideas, but any ground effect or downforce creating shapes on his 1963 or earlier cars wouldn't be one of them. Look at this 1963 photo and judge for yourself. While Jim did a lot of development work on his Chaparral 1, it took him a year to make it even competitive, and the original design wasn't his. It was almost all the work of Troutman & Barnes who built the first cars, body styling was all the work of industrial designer Chuck Pelly. Jim never made any secret of the fact that he didn't have much to do with it, he says that he never thought of his first car as a Chaparral at all. Also, I can't quite understand how anyone could claim that any experiment that had the opposite effect from what was hoped for was "a technical innovation".
McLaren on the other hand achieved exactly what they'd hoped for with their toe in the water experiment with the M6A underside. After the first day of Goodwood testing, Bruce got the times of the bare chassis car, with only radiator ducting fitted, down to 1:16.2. When the body eventually arrived, he was able to lap consistently in the mid 1:14s, a full two second improvement, and rather more than they'd hoped. When Denny had a go, he achieved times a full second faster than that. We all know how successful the car was when raced, so I think that certainly does qualify as innovative.