In the world I am building the people use large powered blimps. They do have a good reason for using these. What I'm Asking is when could people earliest make blimps If planes didn't happen.
Blimps or airships are difficult things. Helium is hard to get, hydrogen is explosive, and hot air doesn't lift much.
Hot air balloons were available from 1783. They didn't have enough payload to carry an engine.
An alternative history could have airships a few years before, but a lightweight steam engine requires metallurgy and precision engineering.