Why didn't Boeing or Douglas make a jet airliner before the British DH Comet?
So did Canada with the Avro  Canada Jetliner, at exactly the same time pretty much.  The Jetliner was complementary to the Comet, being a shorter range intercity airplane.  It made a sales tour of the US in 1950 and was a huge sensation.  Avro Canada was ordered to set the project aside for the  CF-100 All Weather Interceptor program during the early 50s so Avro Canada had to decline US airline interest resulting from its sales tour (and a bizarre Howard Hughes/TWA subplot) and the program never really got traction after that as the company went on to put all its resources into the Arrow supersonic interceptor program by the mid 50s.  Odd that the Jetliner had its legs cut out from under it by government politics (the Jetliner was initially designed to a state-owned Trans Canada Airlines specification, who later back out) and the Comet by a fatal design flaw, so in the end Boeing prevailed anyway.

The main reason for the US lag was that jet engines were considered by private US airlines to be too immature as a technology to be used for commercial travel in the late 40s, so there was little market interest in the US until an actual aircraft was demonstrated (one factor: first generation commercial turbojets with crude fuel controllers, like the Jetliner's RR Derwents, were very difficult to manage).  On the other hand, both the Comet and Jetliner were developed as products for state owned airlines (BOAC and Trans Canada Airlines) wanting to be bold, so they were ostensibly private ventures but with a "guaranteed" state buyer for at least initial production, providing a huge leg up.  

There was no US equivalent in the private airline sector in the late 40s/early 50s, only the US military, where the 707 actually got its start.