In early March I wrote the first piece of this story and explained why I think that a "small widebody" seems unlikely as a 737 successor aircraft.
Hearing Boeing CEO McNerney at the conference call on Wednesday kind of confirms that. He said that the next new airplane will probably first address “the heart of the market,” which is 145 to 185 seats, McNerney said. “Which is not to say that we will leave the (7)57 space unaddressed at all.” (quoted from the seattle.pi blog).
So Boeing clearly will not back away from the "Southwest Airlines" market, which is (as of today) the lower end of that bandwidth, but SWA just this year also ordered their first batch of B737-800, representing the upper end of that "heart of the market".
Not leaving the 757 space unaddressed means they also plan to have an aircraft in the family with 200+ seats. But this would lead to a competitive disadvantage on the lower end, if all family members would have the same wing. If the 200+ seater should have a transatlantic range, the wing has to be considerably larger than today's 737NG wing. That would burden the 145 seat family member with a wing that is too large and too heavy. Trim drag would have to be used during cruise to keep the aircraft flying in level flight with the desired speed, as the large wing produces too much lift - alternatively the aircraft could fly slower, burning less fuel, but cutting productivity.
And by the time the new Boeing aircaft arrives, there could be, apart from the CSeries, another narrowbody - optimised for around 150 seats and probably lower range than the ~3000nm CSeries: a new 5 abreast aircraft from Embraer. With around 2200nm as today's EJets, a specific version of the GTF, the LEAP-X or even a new engine from RR, it could be better than the CSeries and the new Boeing aircraft.
Boeing of course could produce two wings - one for the lower end of the market and one for the upper end - in case of the 787 that strategy failed, but for reasons that do not apply in the narrowbody sector.
About the timing McNerney said: “You’ll hear more from us at the end of the year.”
So, as I wrote in the third part of the story, we will obviously not hear anything spectacular from Boeing at the Paris Air Show - only a "guidance", probably further emphasizing that they lean towards an all new airplane, but not ruling out a reengining at all.
The first three parts of the story are here: