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The first prototype, which sure enough looks like an iPad, was the brainchild of Alan Kay: the Dynabook. This appeared in 1972.
What's that? These are all just concepts or fiction? What do you think design is? Apple isn't fighting Samsung on patents, they're saying Samsung duplicated their look. Clearly, the tablet look has been around for thousands of years. But, if it's real equipment you want, real hardware we've got. The first tablet that shipped commercially, as far as I can tell, was the GRIDPad in 1989.
The GRIDPad was followed by a slew of other unsuccessful tablets, including—oh the irony—HP's first failed tablet 1992's Compaq Concerto, . It turns out the HP TouchPad had failure in its DNA.