Originally Posted by
WHBM
I believe the difference was actually in the tankage, to get a reduced MTOW; the engines were the same, these stepped forward from turbojets to fan jets on both types. The 720 had a lighter internal structure and landing gear because of the lighter weights. The 720B, with the engines off the 707-320B but lighter weights, was regarded as a real Pocket Rocket.
Aer Lingus used 720s (non-fan) on Shannon to New York, and sometimes could not make it and had to drop into Gander.
Because of the lightweight structure and reduced MTOW the 720 was never suitable for conversion to a freighter.
The 720B had 17000# thrust JT3D engines where as the 707-320B/C had 18000# engines. Still a rocket ship.