Does this bike exist?

I'm looking to build a singlespeed for endurance gravel racing (100+ miles).  Durability & comfort are very important to me.  As such, I'm looking for a frame with the following features...

1.  Metal frame.  (No carbon.)
2.  Singlespeed.  (No EBB.)
3.  Disc brakes.
4.  Clearance for up to 50mm tires.

I plan to use drop bars.  So, while I'm not opposed to using a hardtail MTB frame, the geometry must be suitable for drop bars.

Does this frame exist?

Sounds like Surly Straggler. Steel, disc brakes, drop bar, SS, will fit atleast 45mm tires.
If you want bigger the Salsa Fargo is a drop bar MTB design with Salsa alternator dropouts which allow for SS.

Craig

If you can settle with an EBB (it does make the discs easier to deal with) the Singular Gryphon might work.

Check out traitor cycles, they might have some ideas for you. I'm thinking about building up a Slot with Jones bars. I've seen pics of them with drops. 

http://www.traitorcycles.com/2015/Bikes_Slot.cfm?Token={ts_2015-05-28_07:25:34}-86151239f5571c63-F2C4F0B3-B7B8-766F-922DBFB588155D3A 

Soma Wolverine is close too.

Since you're running discs, you might be able to find something that can fit 650 x 50 tires, even though it would normally be sold as a 700c bike.

Ti vaya.....might not take quite a 40mm......but itll take a 43 all day long.  alternator dropouts take care of the singlespeed tension

Salsa Fargo meets all of your criteria.

I love my Fisticuff.  Meets your criteria I believe.  I did a 1x10 but thought long and hard about single speeding it out.  I live in the mountains and wanted to do so single track so the 1x10 won out.