American Airlines' huge order - estimated at $35 billion for a total of 460 Airbus A320s and Boeing 737s - marks its first major single-aisle re-fleeting campaign for almost 15 years.
The deal provides it with inventory to roll over all its ageing, inefficient Boeing MD-80s and 757s, greatly reducing the average age of its fleet and the size of its fuel bill.
The airline's last big single-aisle deal was signed in November 1996, when it selected the 737 Next Generation as part of a long-term "exclusivity deal" with Boeing for more than 600 aircraft, including 75 orders and 425 purchase rights for the 737.
Since then, American has progressively topped up its 737 order book through the conversion of purchase rights, taking its total orders for the type to 207, prior to yesterday's announcement.
Flightglobal's ACAS database puts American's current average fleet age at 15 years, which is one of the oldest among the US majors - only Delta Air Lines has a higher average, at 16 years.
It is no coincidence that both these airlines still fly large fleets of MD-80s.
The new deals take American's firm backlog (when all contracts are finalised) to 566 aircraft, of which 511 are single-aisles.
The replacement of its 217 MD-80s and 123 757s will dramatically reduce the average fleet age - the airline claims it will be less than 10 years by 2017.
All the US major carriers have relatively small single-aisle backlogs - American's was only 51 units prior to the announcement.
However, yesterday's order fest is expected to trigger a series of similar deals in the coming months, as US Airways, United Airlines and Delta are all finalising single-aisle evaluations.
Delta in particular has an urgent need to formalise its fleet renewal plan as it has an order backlog of only seven narrowbodies (A320s), but operates a large number of ageing airliners, including 117 MD-80s as well as 32 McDonnell Douglas DC-9s that it inherited through the 2008 merger with Northwest.