Why Boeing airplane models start with number 7?

I was wondering about the real reason for a long time. Why they name them as 7X7?

Forget all stories and conspiracy theories you heard about the model numbering of Boeing airplanes. There is a logical reason which makes sense, at least at the organizational level...

After World War II, Boeing President William Allen set about an ambitious restructuring of the company, dividing each new department by three-digit numbers. The 300 and 400 departments continued to focus on the company’s propeller aircraft offerings, while 500 came to denote the company's part focused on turbine engine development. The 600s were reserved for rocket and missile products. And finally, the burgeoning jet aircraft division of the company was assigned the number 700.

As the company prepared its Model 700 jet for commercial sales, Boeing’s marketing department swept in and rightfully suggested that the name didn’t exactly roll off the tongue. Instead, the 700 was renamed as the alliterative 707. The 717 followed the pattern, with subsequent decades, bringing the 727, 737, 747, and so on up to today’s 787 Dreamliner.

Click here to read the full story and visit the official pages of Boeing about their commercial airplanes.