It is the antenna for the inflight wifi system. The Southwest aircraft has a Ku band antenna on the top, allowing it to access the internet through satellites. The United aircraft appears to have a similar system.

They are fairings for one thing or another that projects above the wing skin. Part of a gun or ammo feed system, fuel cap, wing fold mechanism, head of a big bolt, whatever. Usually, components are designed in the original versions of the aircraft to fit within the wing profile, but mods and add-ons later introduce bits that project beyond it, so the easiest thing to do is fair it in with a blister.

This cutaway drawing shows it's the airconditioning unit, which would also explain why it has an air intake.

Source - ww2aircraft.net

Zoomed version:

The relevant part of the legend:

Next to the air conditioning unit, the sponsons also house other parts (as pointed out by @sophit in the comments):

• 142: Port entry door
• 141: port entry tunnel and inner pressure door
• 140: Triple twin-wheel main undercarriage bogies
• 135: Sponson mounted cargo winch, port & starboard
• 138: Hydraulic retraction jacks
• 136: Inward opening mainwheel bay doors
• 112: Spons bag-type fuel tanks
• 111: Avionics racks, port and starboard
• 110: Fuselage sponson tail fairing