SilkAir announced they signed a LoI with Boeing for 23 B737-800 and 31 B737MAX-8 with another 14 options. The orders can be converted to other versions.
This is significant as Silkair is the first customer Boeing catched from Airbus. Silkair currently operates a fleet of 21 A320 with 3 more on order.
Until now it was Airbus who draw customers away from Boeing, the first one being American Airlines - prompting Boeing to announce the (then yet unnamed) B737MAX.
There are a lot more airline campaigns where one or the other is trying (or will try) to get a customer from "the other side". Another larger campaign where no decision is made (at least not publicly) is at Pegasus.
It is safe to say that the hard fight between the big two is drawing down prices, making it hard for others to compete - notably Bombardier with their CSeries. On the other hand there is not a single customer for the B737MAX-7 and only very limited success for the A319neo so far, so we cannot say for sure if sales for the CS300 are hindered by Airbus and Boeing. Bombardier recently was successful at AirBaltic. The airline opted for the CS300 instead of the A319neo or the B737MAX-7 (or maybe the Embraer E195).
After the CSeries will have had it's first flight with the first CS100 prototype - still scheduled for the end of the year - we will eventually see more orders for the canadian newcomer. But the price pressure, ignited by Airbus and Boeing, will still be there.