Not forgetting the Saro Princess from Britain and the French Latecoere L.631. I thibk the Hughes H.4 has the largest winspan of any aircraft ever built but the Princess is the biggest Flying boat to fly successfully (ie more than a couple of feet high and more than once) would that be right?
Also, it must be said that Seaplanes are not flying boats and vice versa. A flying boat has a hull whereas a seaplane has floats.
You are right about the flying boat versus the seaplane. It's one of those words that get misplaced every so often, like Motor and Engine being used incorrectly. (Motor = electric, Engine = combustion)
The Princess had a wingspan of 219 feet, where the Martin's was 200 feet. however, only one Princess flew and only then as test flights (something like 40+ flights) where the 6 Martin JRM flying boats were used from the end of WWII through the early 50's, retired and then put back in service off and on for the next few decades. And the two surviving JRMs are still in use today (Mars and Phillipine).
The Blohm Voss Bv238 (not the Bv222 Viking) was a close second to the JRM with a wingspan of 197 feet.
But that would still make the Princess the biggest 'boat' to fly successfully wouldn't it? 40 plus flights without mishap is pretty successful, just not operational.
There was also a scheme to turn the Princess into a landplane which, with the hull planing bottom replaced by a conventional fuselage bottom and undercarriage, rather resembled a six engined C-133. This was halted at the last minute but had it happened wold it have been a unique conversion or have there been others?