Implementation of wideband beamformer for planar array
You need the polar response of each individual array element. That is the transfer function of each element as a function of azimuth and elevation (and potentially distance if you want to operate in the near field). For many arrays the polar function will be the same for all elements, but it doesn't hurt to check.
To calculate the polar response of the whole array you need to do the following


Create a spatial grid on which to calculate the polar response (azimuth and elevation) for the array
For each point on the grid calculate distance, azimuth and elevation of this point AS SEEN BY THE INDIVIDUAL ELEMENT. This is in essence a transformation between two different spherical coordinate systems. 
Look up the single element transfer function for this direction in the single-element polar data. This may require interpolation between measured directions
Apply this transfer function to the input signal of this individual element
Make sure to properly account for the distances. That's typically done by a factor exp(-j*w*(r-r0)), where r is the distance between grid point and element and r0 and convenient normalization distance. A good choice is the distance at which the single element polar was measured.
Some results over all elements
Repeat for all points on the target polar grid