Boeing B-17 Flying Fortress warbird "Nine-O-Nine" at Aurora State Airport on 16 June 2017.  Civil registration NL93012 is painted in small black numbers on the rear fuselage under the horizontal stabilizers.
<a href="//commons.wikimedia.org/wiki/User:Joshbaumgartner" title="User:Joshbaumgartner">Josh Baumgartner</a>
CC BY-SA 4.0
<span class="int-own-work" lang="en">Own work</span>