Boyington says he learned techniques to shoot down Japanese zeros while with the Flying Tigers. The Flying Tigers were taught the techniques to defeat Japanese fighters and bombers by Claire Lee Chennault who is generally considered the father of modern aviation warfare.
The Flying Tigers were actually paid a bonus of $500 for every Japanese plane shot down. This is about $10,000 in 2018 dollars. However, payment was usually delayed until the end of the month due to the shortage of cash.
The film begins showing Pappy Boyington recovering from a leg wound after "shooting 6 Jap planes" in one day while flying for the Flying Tigers in 1941-42. His days as a Flying Tiger would be mentioned throughout the series, although the year is usually mistakenly mentioned as 1939 or 1940, not 1941 or 1942.