I found this explanation on the website "www.tradeslang.com"
******************************************
Much like the invention of 3M Post-it-Notes, BX was an unintentional invention. During a production run of GREENFIELD flexible conduit, a machine malfunctioned and partially pulled some packaging twine through the conduit as it was being manufactured. Knowing that electricians in the field pulled wire through their flexible metal conduit, the Greenfield Company recognized the potential of this product and perfected the process of applying armor around electrical wires. There are several stories about where the term BX came from but my favorite is that “B” stood for product line B and the “X” stood for experimental. It was recognized by UL in 1899 under their Standard #4. The picture shown here is from the 1924 B&R Electric Supply catalog. When first invented, it consisted of a hot and a neutral Type R rubber insulated wire with a cotton-braid to protect the insulation and was sold in wire sizes #18 through #10 on the B&S or (AWG) gauge. Later innovations included a flat wire strip or bonding wire, individually paper wrapped conductors and 60 degree type T wire. For wet locations a lead covering was wrapped over the wires and was called “BXL”
***********************
