Northwest Airlines Flight 421 was a scheduled passenger flight flying from Chicago Midway International Airport to Minneapolis-St. Paul International Airport. On a stormy evening, the Martin 4-0-4 aircraft faced severe weather with thunderstorms and turbulence that led to its demise. The pilot, Captain Robert W. Smith, struggled to maintain altitude as the plane encountered difficult conditions during its approach to the destination. Tragically, it crashed near Fountain City, Wisconsin, in the Green River Valley. Eyewitnesses reported hearing a loud explosion, which led to immediate emergency response efforts in the area.

Rescue teams found no survivors among the 34 passengers and 3 crew members. The wreckage was widespread, emphasizing the intensity of the crash. In the aftermath, investigations revealed that the aircraft had likely been caught in downdrafts from the storm, with the weather proving overwhelming for the crew's attempts to navigate through it. The incident prompted federal authorities to closely examine weather-related safety protocols in aviation.

This crash not only resulted in the tragic loss of life but also sparked renewed discussions regarding the implications of flying in adverse weather. Northwest Airlines, like many carriers at the time, faced scrutiny over operational protocols and pilot training for extreme weather conditions as part of the investigation that followed. Such tragedies often lead to changes in industry standards, and this incident contributed to enhancing aviation safety measures.