This photo of the [XB-70 Valkyrie](https://en.wikipedia.org/wiki/North_American_XB-70_Valkyrie)'s main gear shows a small wheel between the two main wheels:

[![XB-70 MLG][1]][1]<br><sup>Courtsey of [AirPigz.com](http://airpigz.com/blog/2012/1/29/day-2-airpigz-air-force-museum-meetup-1-28-12.html)</sup>

This smaller wheel doesn't touch the ground, though I can see where it might as the side walls of the main tires compress under landing weight. 

The AirPigz site promised an explanation, but none has been forthcoming (in the 4+ years since that was posted), and I couldn't find any information on it on Wikipedia, either. Does anyone have any information (real or speculative) on what it's for?


  [1]: https://i.sstatic.net/zwzkn.jpg