I think it might be a recognition model (likely American made to train troops to identify aircraft) that someone painted up and copied some random Japanese kana onto along with some excessive markings to make it more interesting (and "more Japanese" as Paul already remarked).
No Japanese person painted it, I can't say for sure it couldn't be a Japanese made recognition model since I've never seen one and don't know what they look like. Either way I'd still count it as a neat bit of "trench art".
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