He does say (hence why I titled it Target Tug)

The job of this plane was to pull the streamers that were used for target practise by our fighter pilots and bomber gunners in training. I recall these souped up hummingbirds pulling the target streamers for our AT-6s in the skies over Texas. And I recalled wondering at the time what would happen if our aim was so bad we plugged holes in the tow plane instead of the streamer.

The 100mph figure became important later as, during a moment of hedgehopping to relieve the boredom of the ferry flight he forgot to keep an eye on the fuel and one tank ran dry starving the engine. Whilst attempting to get it restarted the high stall speed of the plane became of everlooming importance to him, as did the proximity to the ground.