So make it if 100,000 lb is dropped on the runway it will be shut down for 20 min.
I went into the arena setup area in offline mode, checked the 'damage' required for runways and it looks to be 125k, if not mistaken.
125k ...
1 b29 with 40 500s (20k) x3 is 60k so 2 formations of b29s and 1 p47 or a medium bomber.. to take out a runway for 20 mins
OR
You use 1 formation of b29s to take out about 3 bases worth of hangars and towns (WF).
Seems like a waste to me.