A very powerful technique that has come up, as digital painting and designing has become more and more prevalent in the concept artist industry, is the usage of 3D in paintings and concepts.
3D renders provide a fantastic plate to paint on, reference model figures that you can design on top of, automatic perfect perspective for scenes, and tons more. They can also offer some great realistic texture and models for your scenes.
This is an example where I used 3D in my painting. The painting was for a trailer music cover album artwork. I designed the 3D spaceship, exported the .obj file and rendered it out in Keyshot. Once I had the renders of the spaceship, I imported it into Photoshop and started compositing it in the landscape and painted over it.
Here is the modelled and rendered spaceship with 3 different materials so I can composite them later in Photoshop: