The story I heard was that Rubens was originally developing a script around Pee-Wee as a Pollyanna type character. He was walking across the studio backlot and

The story I heard was that Rubens was originally developing a script around Pee-Wee as a Pollyanna type character. He was walking across the studio backlot and noticed a lot of people use bicycles to get around the lot. He requested his own bike, and the studio gave him an old (40s?) Schwinn which he just fell in love with. He realized Pee-Wee would too, and it became the MacGuffin for a new script. The original Pollyanna script was eventually developed into the sequel Big Top Pee-Wee.