So, United Airlines was to be the absolute deciding factor and by apparently offering the airline large numbers of 727's at giveaway lease terms, they secured an order for 40 737's plus a further 30 on option.
It is a well known fact that the prototype 737 had various problems. Drag figures were worse than expected and the thrust reversers were ineffective. The ALPA insisted on a 3-man crew adding 50% to crew costs compared to the DC-9.
Had it not been for United Airlines and the Boeing's lease terms on the 727's would the 737 have been a dead duck in the water?
Below, the very first 737-100 in airline service with Lufthansa
Photo © Fritz Gratenau
Another early Lufthansa 737
Photo © Nils Rosengaard
An early United 737-200
Photo © AirNikon
Another early United 737-222 in an updated colour-scheme
Photo © Tom Hildreth