How to delete all photos in the camera roll in iOS 8.x?

I have started using auto backup using OneDrive and it works like a charm. The only problem is that the pictures are left on the phone after being backed up.

Is it possible to empty the camera roll or in some way mass delete photos? (Even with an app)

Edit: I stopped using OneDrive because it kept losing pictures. Switched to Google Photos and it seems to work correctly.

Edit: And now Google Photos as a button: "Free up storage" which deletes all the photos that have been backed up.

If you have a Mac, connect your iPhone to it.

There is a build in app (from Apple) called Image Capture in your Applications folder.

Use it to bulk delete Photos on iPhone.

You can use Image Capture... However, if you see a cloud next to the name of your device then you need to disable iCloud Photo Library before the delete all button will appear.

iPhone: Settings -> Photos & Camera -> iCloud Photo Library off Settings -> Photos & Camera -> My Photo Stream off

Then, try Image Capture again and the delete all button will appear.