Boeing delivered its 8,000th B737 earlier this week. The aircraft, a 737-900ER, was delivered to United Airlines. The airline has since 1965 taken delivery of more than 550 B737s, of several variants. Current “Next Generation” 737 models, including the 737-700, 737-800 and 737-900, superseded the “Classic” models. Now Boeing is developing the 737 Max series, with new engines, wing tips and cockpit electronics, although most other aspects of the aircraft remain essentially the same.