The strips you secured to the bottom are the cause of the warping. The main boards can't expand and contract because they are fixed to the strips at a few points. If the boards were just glued to each other then it would expand/contract as a single panel. This is the same problem you face with tabletops, hence the need for connections that allow movement.
Also, not sure if you did in this case, but alternating the boards up/down/up/down will help prevent this type of warping as well.
At this point I would try removing the strips to see if it flattens out. If it does, and you wanted to keep the strips for support, the solution would be to cut keyways (or large holes) where the screws attach so that the panel can move relative to the strips.
Placing cauls across the panel while you glue and clamp it will also help keep it nice and flat as the glue dries.