The 'major airline' bit of Boeing's press release strikes me as well. I have a hunch that it could be ONE Chinese airline and have already suggested Air China - just a hunch, no insider info or anything. Of course, Air China's subsidiaries could be taking many of these MAXs as well as Air China mainline, but the group as a whole could be described as one 'major airline', couldn't it?
Maybe it is Air Asia (lol) or someone else holding off to make a big splash at some specific big event (airshow eg. Dubai later this year?) because that is their nature and modus operandi - big splashes at big events? Personally, I can't imagine Air Asia ordering them, but you never know I guess... Norwegian and American and Lion Air and others will all operate neos and MAXs in harmony soon ...
What about easyJet? Could they switch their neo orders to predominantly A321neos like they have already started doing, and slide the MAX8s in underneath capacity-wise like a few other airlines have done/will be doing? Wizzair could also be considered as a potential operator of the MAX using the same scenario.
Ryanair? Unlikely with their recent small MAX top up order.
Norwegian? Possible I guess but again a recent small MAX top up order was made.
AirFrance/KLM? Not this many, surely... I would think that IF the MAX was to be ordered by the Group, it would be for KLM/Transavia only - and certainly not for 125+50 in one hit of anything!
Vietjet? I don't know enough about them to pass opinion but I guess with their aggressive expansion plans they would have to be considered - was their 100 frame MAX order from a little while ago completely finalised? Could they be considered by Boeing to be a 'major airline'?
GOL/Copa/Azul/Avianca/LATAM? No, no, no, no, no ... in my opinion anyway. All of these are either already wedded to the neo, are too small to order 125+50 frames, or other reasons...
Qantas? No, not yet and way to big an order to replace just 75 x 737-800s when you remember that there are dozens of neos already on order for the various Jetstars (or so QF says).
Jet/SpiceJet - already heavily discussed above and I just can't see IndiGo or Goair ordering MAXs as well as neos despite others already doing so. Again, just personal opinion.
It could of course be Southwest or American or Delta - but I think unlikely for reasons suggested already in this thread. Has United been discounted?
PS: just a further thought ... Boeing may be referring to the mystery customer as a 'major' airline as an ego boost of sorts, if in fact the airline isn't a usually-considered 'major' airline... though the quantity of frames involved in this order (125 + 50) does indeed make it a 'major order' from whomever it is!
I just wanna live while I'm alive!