The only NFL team that does not have a logo on the helmet is the Cleveland Browns. Instead of a logo, the helmet is simply colored orange with alternating stripes of brown and white lines going down the mid-line.
The helmet without a logo has been a mainstay with the Browns organization through two stints in Cleveland. The Cincinnati Bengal's helmet has a pattern similar to the stripes of a tiger. While this color scheme may differ from that of many other helmets, such as a cartoon figure of the mascot or team insignia, it may be considered a logo.