Not content with the MOAB the USAF is developing the larger Big
Ordnance Penetrator (B.O.P.); despite its size, this isn't the largest
conventional bomb developed by the USAF (actually USAAF)- the 43,500
lb T-12 Cloudmaker that didn't make it into WW2:

http://www.stelzriede.com/ms/photos/misc26.jpg

Rob