If you wish to create an App for in-house use only (not to be sold published on the App Store for download) you will have to buy the Apple Enterprise Developer package, which allows you to deploy the app to your business devices on your own.
For a more detailed overview take a look at:
Remember that the Enterprise Account requires you to distribute the app exclusively within company members.
Steps to follow
1. Register on the iOS Developer Enterprise Program
2. Follow instructions under the deployment section
For a detailed guide on what to do and how to do it, see the dedicated Apple Help Page.