New answer
Thanks for the comment, @ShawnAnderson. You are absolutely right. All you have to do is remove the panel-title class and h1 will work.
<div class="panel panel-default"> <div class="panel-heading"> <h1>Panel title</h1> </div> <div class="panel-body"> Panel content </div> </div>
Fiddle: https://jsfiddle.net/DTcHh/29507/
This answer is preferable to my original answer, since you do not need to override any bootstrap classes.
Original answer
This small amount of css will do the trick if you enable it after bootstrap.css.
CSS
h1.panel-title { font-size: 36px; } h2.panel-title { font-size: 30px; } h3.panel-title { font-size: 24px; } h4.panel-title { font-size: 18px } h5.panel-title { font-size: 14px } h6.panel-title { font-size: 10px; }
Results:

This works, but I would like to know if anyone has any better ideas.
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