This is pure speculation on my part but I think Boeing are playing out a strategy to leapfrog Airbus.
When the 320 series has Airbus fully committed and the 737NG is running out of steam, I think they will introduce a radically new design to replace the 737 series. Airbus will either have to convert 320 orders to a new generation of aircraft or just fullfill their existing orders. It will either be very costly for them or they will have a lot of disgruntled customers.
As for the A3XX, they will be popping corks in Seattle if it is ever launched because it will destroy Airbus, or if the Govt's step in there will be some very unhappy taxpayers who will be unwilling to continue to pour money into Airbus. My guess is it will never get off the drawing board because Airbus is well managed and they must know the risks far outway the likely gains.