Easy access to a gas with a lower density than the rest of the atmosphere would help. In our world the available alternatives for this are:

* Hydrogen which is easy to manufacture in large quantities but [has proven to be too unsafe for use in airships](https://en.wikipedia.org/wiki/Hindenburg_disaster). 
* Hot air which is free and has the great advantage that its density can be adjusted by adjusting its temperature which allows to regulate the height, but requires a constant heat source which makes it resource-intense.
* Helium which is both save like air and has a density which is almost (but not quite) as low as hydrogen, but is very expensive to manufacture.

For further reading, I recommend [the Wikipedia article on lifting gas][1].

A society which has easy access to a gas which is lighter than air and safe to handle would likely develop balloon/blimp based transportation much earlier than aerodynamic flight.

A possible world could be one with a layered atmosphere where the lower layers are breathable air and the upper layers are pure helium. When the world has mountains which are so high that they reach into the helium layer, those could be used to harvest the helium and use it for airship construction. When there aren't many of such mountains, they would be of large economical value.


  [1]: https://en.wikipedia.org/wiki/Lifting_gas