The ultimate [the very ultimate] in aeronautical-engineering monstrosities:
design and build a flying aircraft
with the weight of supertanker?

For example: an ocean-cruising supertanker, 1500 feet-long, with a beam of 250 feet, displacing 500,000 tons,
is almost 1000 times heavier than the heaviest aircraft ever built and flown [the ANTONOV 225].

The Antonov 225 has a max. takeoff
weight of 600 tons; it has six huge engines providing thrust to give the
aircraft a max. speed of over 500mph.

Someone on habitablezone.com may make a snide remark about a 'flying supertanker' requiring 1000 times as many engines as the Antonov 225 has.

NO!...You don't need 6000 jumbo jet engines on a 'flying supertanker'.

Actually, since you only need a TOP SPEED of just over 100mph to get 500,000 tons of hardware, fuel, and persons airborne [and stay airborne for the length of the flight] you need only
6000 jumbojet-engines/[500mph/100mph]^3
which works out to 6000/125 jumbo jet-engines = 48.

48 jumbo jet-engines would provide you
maybe a max. of over 2000-2400 tons of thrust. No where near enough for a cruising speed of 500mph, at 30,000ft, for some 500,000 ton FLYING MONSTROSITY ...but just fine for a cruising speed of 100mph, at maybe
10-20 feet about the ocean waters.

Now I used the phrase 'flying supertanker', not because I'm interested
in transporting crude-oil by air at 100mph, but because a FLYING MONSTROSITY
that big [1500ft long fuselage, probably
four pairs of lift-wings; each pair over 1000ft long] could hold 200,000 people
crammed into its holds. A flying army
on one plane?

OK! You can stop laughing now!

Jacob