[lmm]

I don't see that the engineering is hard. You could just take an existing design and replace most of the cabin with batteries.

Building an economically viable mass-market aircraft with a 70% mass fraction might be another question, but if we're just talking about the engineering challenge of building it at all I don't think that's hard?

[xgbi]

And on top of that, "refueling" wouldn't cost the 250k figure that usually gets thrown in comments. I imagine it being more in the space of a 1-2k$ ; in which case the reduced passenger capacity might still be worth the premium in batteries.