However, from what I've read, it sounds like radar reflectons from the doors themselves are dwarfed by radar signals bouncing off the upper bulkhead of the bomb bay. If they made the side bulkheads of the bomb bay at 90 degrees to the upper bulkhead, then an incoming radio wave bouncing off both the upper and side bulkheads will leave anti-parallel to its incoming direction... right back at the sending radar. (This geometric identity is used on the laser reflectors left on the moon. They send the laser right back at the sender without having to track the Earth.)

Though, maybe the reporting I've read is just imprecise, and maybe the doors do have a larger radar cross-section than the bomb bays themselves.