I used a single retract servo at the front of the bomb bay. The servo sits so that the output shaft is runs parallel to the fuselage. I fixed a large control horn to the front edges of each door and have a steel rod extending up from these horns. The servo has a custom arm that I made that is approx 2 inches long (for my doors you need 4 inches of travel to fully open them). When the doors are closed, the servo arm points up, when it opens, it rotates 180 degrees down. The benefit of this arrangement is that when the doors are closed, there is no load on the servo motor, as the arm and servo body supports all of the weight of whatever is in the bomb bay (obviously the servo must be mounted very strongly). Here is my poor attempt at a picture of this arrangement... (looking from the rear of the bomb bay toward the front. Servo arm swings clockwise.)

BTW, do all of this at the front of the bomb bay to avoid having your cargo tangle up in the mechanism during release.