Why do Boeing Passenger planes end with the number 7? (i.e. 727, 737, 747, etc.)

Hello,

The following comes from "Why Do Dogs Have Wet Noses? and Other
Imponderables" by David Feldman (HarperPerennial, 1990):

Since the early days of aviation, Boeing has assigned three-digit
codes to its airplanes. The 100 series was early planes; the 200 and
400 series military planes; the 300 series earlier passenger planes;
the 500 series industrial products; and the 600 series missiles. The
first plane in the 700 series was called the 707 simply because the
name was catchy. The second version of the 700 series passenger jet
was the 727, followed by the 737, 747, etc...

In case you are wondering, there is a Boeing 717 -- it's a aerial
refueler the military calls the KC-135.

I hope this answer meets your needs. If not, please request
clarification.

Thanks for the answer, That was my guess.  The 717 is a renamed MD-95
which Boeing still produces and is a passenger plane.

My apologies. You are correct that Boeing now has a 717 passenger
plane. However, the KC-135 also was called the 717:
<http://www.boeing.com/companyoffices/history/boeing/kc135.html>.