Hi Anton,
You can create a Mendix deployment in a no-internet environment. Here I assume your development machines do not have internet access.
For your development machines:
In your deployment environment:
Additionally to erics’ answer;
You mention you do have a no internet infrastructure.
My question: does this relate to the application server and users or also for development?
Because I can understand that the usage of the app and thus the deployment infrastructure being in a closed environment. But that doesn't mean by default that development must be in the same closed environment. You can easily develop an app using the online services like Sprintr, appstor, TeamServer etc. while deployment is done through package creation and passing that into your closed infrastructure.