This interesting little tidbit was sent to a friend of Fighterpilots.Net  by retired Navy Royce Williams.

Subject:  100 missions north...

Bob Swartz, served as the 7th Air Force Scientific Advisor to General Momyer. I asked Bob, if he had any idea how 100 missions over North Vietnam came about. He gave a surprised look, and
stated yes he did.

General Momyer, realizing loss rates were high, ask Bob to do an analysis and make a recommendation of how many missions should count as a completed tour. Bob said he looked at WW II and Korea , but used WW II data to come up with 100, figuring ultimately that two times 50 was about right.

I ask him if he knew where the expression "There ain't no way" came from, and proceeded to tell him that at the time General Momyer made the decision, F-105 pilots at Takhli (or Korat) could expect over 100 missions North to be shot down one and one-half times and picked up once. At the time, late 1965, no pilot had yet to get to 50. Bob was not surprised that the overall attrition rate of F-105 pilots from 1965 to the halt in early 1968 was 28 percent, which does not include pilots who were shot down and picked up.

As I recall, the F-105 aircraft attrition rate was 42 percent. <:)