Out of curiosity, I Googled Stuka, and the consensus seems to be that the decision to stick with fixed landing gear was driven by the need for structural integrity. These planes were designed to dive at 80 degree angles and the abrupt pull-up after bomb release demanded very strong wings. Wing strength would have been compromised by the addition of recesses for retractable landing gear, so the wheels were mounted on fixed struts. The reverse-gull-wing design was adopted so that the struts could be as short as practical, which made them less prone to flexure on landing and take-off in rough airstrips. The struts and wheels were skirted to reduce drag in the air, and also to make them less likely to be fouled by vegetation, etc in the rough makeshift airfields that the Stukas were deployed from. Since their landing gear was very simple and rugged, the Stukas could be based very near the front lines and could fly more sorties per day than planes with retractable landing gear that had to use more conventional airstrips. This gave better air-support to the ground troops, more opportunities to destroy bridges and attack convoys, etc, and saved in fuel (very important!). When the cowlings around the wheels were reduced in size (later in production) they were often removed in the field because mud could foul them and prevent the wheels from spinning freely.