The only place that you will be able to download and access the Microsoft Flight application will be through the official Microsoft Flight site and will be hosting free downloads of the application. I doubt you’ll be seeing the game on Steam any time soon, this is Microsoft we are talking about.
The other piece of news is that when the game launches on the 29th, a piece of DLC will already be available called “Hawaiian Adventure Pack” and will feature the “remaining Hawaiian Islands, a new plane and 20 new missions” and will be available for $19.99. This makes me question how much content will actually be in the free download and how long before you want to download the DLC pack. Other DLC will be releasing (I assume frequently) after the game launches.
Check out Microsoft Flight on Feb 29th for free on the official site.