One day in the early 1990s, as Simpsons writers Bill Oakley and Josh Weinstein were working on a script in their office, Matt Groening appeared at the door and blurted out an idea. “We should do an episode like ‘Who shot J.R.?’” Weinstein remembers the creator of the show saying, “but with Mr. Burns”

Source: The making of “Who Shot Mr. Burns?” (via Zach Morek)

Lots of interesting history here.  They took it so seriously, but it still ended up being funny!