Let alone the problem of making anything, you would have an issue designing the aircraft.
If computers don't exist, do you design for missile defence? Chaff? What about radar? ECM? ECCM? What kind of offensive weapons?
Would the "computer" in the V bombers be OK? It was analog. If it would be OK, then use analog computers to design the aircraft first. Or just design a digital computer from scratch. Mr Sinclair did it.
Do you go for some kind of turbine engine, or a piston engine? I imagine it might be quicker to design a turbine than a piston engine from scratch.
As said, you can't "unlearn" something.
Interesting conundrum though!