For years, from the early 1990s until 2007 and the pull out of NFL Europe, the NFL operated some form of American football league in Europe. Originally started in 1991 and known as the World League of American Football with eight teams based both in Europe and North America, the league operated in the spring.
The first winner was the London Monarchs. As a spring league, it offered NFL teams a chance to evaluate talent and bring players in before training camp that very same year. And there were a number of quarterbacks who were able to develop and grow under that system and enter the NFL with game-playing experience. Kurt Warner, Jake Delhomme and Brad Johnson are three who spring to mind.
In this article, Rick Gosselin of the Dallas Morning News (re-posted in the Sports Carolina Monthly) explains how important NFL Europe was in supplying top quality quarterbacks for NFL teams.
https://www.sportscarolinamonthly.com/nfl-pulling-plug-european-league-made-harder-find-quality-backup-qb/