Wasn't the first tank (and helicopter) conceived by Leonardo da Vinci? Do mobile warring cities count as tanks? I think Edgar Rice Boroughs first thought them up in the John Carter of Mars series. H G Wells's Martian tripods could be argued to be mechas, which are walking tanks. I'm trying to think if there's an early SF story that predicts tanks with track-like locomotion before the real deal was first deployed in WW1. If there is, it'd probably be written by Wells. I haven't read The Land Ironclads (1903) so I don't know how its tanks compare.
ETA: Wells's land ironclads (more APV than tank?) used eight pairs of Bramah Diplock-invented pedrails (1903) whereas the first tanks in 1916 used continous tracks, first invented by David Roberts in 1903 (patented 1904). I'm not really a military buff so I can stand to be corrected. I've seen some film of (Israeli?) modern tanks being demonstrated that were really motoring, almost bouncing, over rough terrain at 40-50 mph. I'm not sure how long they could sustain that. Remotely operated or autonomous drone vehicles will supplant them I expect. Terminator world, here we come.
