Fill a corner with a curve? I'm trying to make a curved transition between some perpendicular planes (mocked up here with Photoshop):


What's the best way to go about this? Perhaps creating some kind of curved edge? I tried using a bevel, but nothing happened.

File here if it's useful: https://drive.google.com/file/d/0B3TF2HeeQRyWRlRyQUcyMjhtaHM/view?usp=sharing
You can use the bridge in the loop tools addon with a negative value in the cubic interpolation.