Richland Bombers!
During the war, about 51,000 workers at Hanford Engineer Works in Richland, WA donated a day's wage to the war effort. "A day's pay to send a bomber on its way" was the slogan. On July 23, 1944, the aptly named "Day's Pay," a B-17G Flying Fortress, was christened and would go on to fly over 60 missions in Europe.
Thereafter, Richland High School (then "Columbia" High School) officially changed it mascot to the "Bombers" to honor the town's sacrifice for the war effort. And probably they also realized that they needed to shed the previous "Beavers" mascot.
Somewhere along the line, the mushroom cloud became the main logo (because Hanford produced the plutonium for the a-bombs dropped on Japan), but they've been incorporating the plane into more of the visual brand.