United Airlines is buying 150 Boeing 737s, planning to use them to replace older planes that are not as fuel efficient.  

 The order announced on Thursday includes 100 of Boeing's new 737 Max 9s.

That's a new version of the plane that will have new engines and other tweaks to cut how much fuel it burns. Those planes start arriving at United in 2018.  

 United is also buying 50 of Boeing's current 737s. Those planes begin arriving late next year.  

 The 737 is a workhorse of domestic flying.   

 The order is a win for Boeing Co., which has been hoping to boost orders for its new 737 Max. 

 The planes would be worth more than $14 billion at list prices, although big airlines like United don't pay list prices.