To use more technical language, they can be called landing gear fairings, but my dad (who was there when they were in vogue) called them spats, and so do I.
Spats came into vogue in the late 1920s when aerodynamic streamlining became an important design consideration; less drag meant higher speed and/or better fuel economy and/or longer range. But fixed landing gear of any kind add to drag.
There are basically two cures to this problem. One is installation of landing gear that retract into the wings, fuselage or a wing-mounted pod of some kind. But retractable landing gear are heavy because they require mechanisms, additional parts and perhaps tanks of hydraulic fluid. In the early 1930s this additional weight, coupled with comparatively low horsepower motors of the time, could result in reduced speed or range. Retractable gear did not become common until the late 30s when more powerful motors were introduced.
The other solution was to retain fixed landing gear, but add streamlined fairings to reduce (but not eliminate) the drag. This was a tricky business because the fairings themselves added weight to an aircraft. When carefully designed, such spats apparently resulted in net performance gains; otherwise, they wouldn't have been so common.
Spats are not a 1930s thing. Light aircraft of today often have fixed landing gear with spats because their designers decided that they represent the best compromise when dealing with the factors of cost, weight, power and performance.
Below are examples of aircraft from the heyday of spats.
Gallery
The F4B was the last production fighter for the U.S. Navy from Boeing. Its most advanced aerodynamic feature is the Townwend Ring covering the motor. Otherwise, its streamlining is little better than that for Great War fighter craft. Note the the landing gear design, typical of what spats were intended to cure.
America's classic "pursuit" plane of the early 30s, the "Peashooter" (its informal name) was a combination of old and new design features. On the old side were externally braced (with wires) wings and the open cockpit (that Air Corps pilots insisted was the only way to fly). New was the use of metal construction. The landing gear spats fall in the middle: they were an advance over exposed landing gear, but not the ultimate solution of retractable gear.
The Shrike was an Air Corps attack plane of the early 1930s. Besides streamlined spats, the plane sports bracing struts from a point above the spats' roots to the fuselage. There are support struts for the spats as well as wing bracing wires. Overall, a pretty draggy airplane.
This was an advanced design for its day. The spats are huge, but their horizontal cross-section was probably something like a symmetrical wing profile -- highly streamlined.
This French airliner has a dainty passenger compartment that, combined with huge spats, creates an ungainly looking aircraft.
This was Supermarine's entry in an early 1930s fighter competition. The designer was Reginald Mitchell who a few years previously graduated from designing pretty ugly flying boats to penning the sleek winners of the Schneiner Cup, one of which raised the absolute world speed record to more than 400 miles per hour. Following the 224, Mitchell designed the famed Spitfire, Britain's mainstay fighter during World War 2. Given the Schneider racers and the Spitfire, it's puzzling that Mitchell and his team came up this rather awkward design that never saw production.
The logic might have been using inverted gull-wings so as to reduce the height and size of the landing gear and the spats; larger spats would have added more drag. The real answers were retractable gear plus an enclosed cockpit, and the Spitfire had these features.
The plane shown here is a prototype of a version later sold to China and Thailand: note the Chinese markings. Hawk 75s exported to France had retractable landing gear, as did the version sold to the U.S. Army Air Corps as the P-36. The spat design here is minimalist to the point of having semi-exposed wheels.