Over the next 20 years both the European and the American aerospace giants forecast that China will become their biggest single market due to demand for new aircraft by Chinese airlines keen to meet the rising middle classes' desire for air travel. Boeing estimates that China will need 6,810 jets worth $1 trillion over that period. The state-owned Commercial Aircraft Corporation of China (COMAC) is eager to supply some of those planes.

This puts Airbus and Boeing in a tricky spot: "Their problem is that their biggest customer wants to become their biggest rival," says Michael Goldberg, a partner with Bain & Company.