A WIX member used the term 'mainstream' to discribe aircraft types that don't get as much
airplay as the usual popular types. I like that definition. Sounds about right to me as well. Many served a great purpose and had a long service life, some not so much, but all were/are unique and special in their own way. Many of these types still survive today and it's always nice to see some of these guys around flying still. So here are just a few of the thousands of great creations that could always use some more attention IMHO. Source, San Diego Air and Space Museum Archives.
Part 1

P-26A of the 94th Pursuit Squadron 1st Pursuit Group USAAC Edit: this image is actually pointed out as being a model, could have fooled me, very nice job. You get the point though I'm sure.

P-26A's which hopefully these are the
real deal and not a plastic model as the one above.

Seversky P-35's assigned to the 27th Pursuit Squadron 1st Pursuit Group based at Selfridge Field Mt Clemens Michigan ca 1938-1939

Vultee YA-19 No 5 of the 17th Attack Group March Field Calif Sept 12 1939

O2 Owl I believe is the correct name

Cessna AT-17A from Stockton Field at Mather Field on May 9 1942

Douglas B-23

Lockheed R5O-14250 staff transport for the Secretary of the Navy At San Francisco on August 4 1941

Ryan PT-22's

Ryan PT-21

Douglas C-39 transport

Ryan STM-S2

Curtiss SNC-1

C-45G

Lockheed Electra East Boston 1940

UC-43

AT-7 42-2428 written off 6-24-43 while assigned 8th AF Bovingdon Station

Beech UC-45F

Howard VR2