Little-known fact: the iPad came out after the iPhone, but Apple actually started work on the tablet first. Steve Jobs said so a couple of years back at AllThingsD's D8 Conference. Now, we have photographic evidence, courtesy of Apple's never-ending legal battle with Samsung.
Court filings snatched up by NetworkWorld and BuzzFeed show a very early iPad prototype, dubbed simply "035," that purportedly dates back to 2002-2004. NetworkWorld has a whole bunch of black-and-white shots, but BuzzFeed's pictures are in full color—and they include an iPad 2 for reference.
The 035 prototype is quite a bit bigger than the existing iPad. It's almost an inch thick, and I'd guess it has at least a 12" display. In many ways, the device looks like a monobloc iBook without a keyboard or touchpad. Apple's iBook G4 had the same thick frame, rounded corners, and white plastic case with a plain logo on the lid.
Around the front, the 035 seems to lack a home button, and the border around its display is much thinner than on the iPad. That thin border might have made it less convenient to use, since there would be less room to rest one's thumbs when simply holding the device without touching the display.
Too bad none of the images show what software the 035 prototype was running. Steve Jobs said Apple's engineers had multi-touch input and inertial scrolling working pretty early on, but it wasn't until 2007 that iOS premiered on the original iPhone. Perhaps the 035 was simply running a modified version of Mac OS X on top of a regular PowerPC G4 processor.