The Western Flyer from Tom Bihn is a small duffel/backpack conversion travel bag designed for quick trips or minimal travelers.
Other Bags to Check Out:
Expensive, but it's got an amazing warranty. A simple, understated, powerful bag that will gladly see you through anything in life.
A very innovative backpack for either large daily carry or serious travelers who need one bag for both travel and daily carry. This is a winner for me in the medium size 1-bag travel category.
The Farpoint and Fairview 40L travel packs from Osprey are solid one bag travel bags with a little less thrills than other bags in similar sizes.
The 50L duffel from Aasgard is a very exciting idea — massive space in a weatherproof simple duffel with backpack straps. (The shoulder straps could use an update though.)
The Wilderness Rucksack from Tanner Goods is a gorgeous, artisan, hand-shucked heirloom bag that your grandpa would be proud of (whichever one of your grandpas was more badass).