Is it possible to develop private apps for iOS (iPhone/iPad) and Android?
With "private apps" I mean apps that do not appear on the public app store, but can be downloaded and used only by selected clients.
Most Android devices can install application simply by opening an
apk file in the browser. You can use self-signed certificates, so no problem there.
It's more difficult with iOS applications. Everything you do there must go through Apple. Apple development site has lots of information about the different ways to do that but mostly it reduces to the following 3 types of distribution.
You can develop and distribute private apps among selected clients.
In iOS, ad-hoc distribution is there, but you'll need to hook up your mobile device to a Mac OSX. Ad-hoc distribution is possible through Xcode, iTunes and Configurator. There are other methods that allows you to deploy apps remotely.
VPP for Business: You can develop apps with your developer account. While distributing, yo'll need to choose an option to distribute the app privately. Now, provide the VPP Apple ID of those (Organization) you need to send apps.
Mobile Device Management (MDM) and Mobile Application Management (MAM) software: MAM and MDM software allows you to bulk push apps to devices (apps from App Store can also be pushed). You'll need to enable supervision to silent push apps to devices. For that, you need either a (Device Enrollment Program) DEP account or Apple Configurator 2. DEP needs D-U-N-S number and Apple Configurator needs you to hook up your device with a computer running Mac OSX.
In Android, develop and upload any app to, say, a website, just turn on "Unknown Sources" in settings, download the app and install it right away. You have two other options too.
Google Play for Work: This is a private store where you can add your private apps to. In the Play for Work console, just add the persons you need to download the app, and they'll be able to download these apps from Google Play installed on their device.
Mobile Device Management (MDM) and Mobile Application Management (MAM) software: MAM and MDM software allows you to bulk push apps to devices (apps from App Store can also be pushed).