Technology advances, but most products aren't cutting-edge due in part to the timing of development cycles. Then there are new products that have obviously retrograde features, a prime example being the Curtiss Condor airliner that first flew early in 1933.
In order to explain the Condor, I cooked up the following photo essay:
Photo Essay
The photos above are of the Condor II, of which there were several variations among the 43 that were built (note the difference in the engine cowlings between the plane in the top image and the others). Its first flight was 30 January 1933.
Note that the fuselage is rounded and has a somewhat streamlined appearance in line with early 1930s aircraft, though it isn't of all-metal construction which was becoming universal for larger airplanes. What is strongly retrograde is the fact that it is a biplane with wing struts that add to the wind resistance. The Condor II was a slow aircraft compared to other new transports such as those mentioned below. Its commercial advantage was that its large fuselage could be configured to include sleeping berths, a selling-point for coast-to-coast flights; previously, transcontinental passengers would fly a few daylight legs and switch to passenger trains for overnight legs of the trip. Apparently American Airlines felt that eliminating this transportation mode-switching compensated for the slow speed of the Condor.
This can be considered the first modern airliner. Its first flight was 8 February 1933, a few days after that of the Condor II. Compare it to the Condor.
There were earlier Condors, one being the Army Air Corps B-2 which entered service in 1929. It was a primitive design based on an early-1920s bomber.
The Condor I was known as the Condor CO or Condor 18, the Condor I appellation is retrospective to distinguish it from the later Condor airliner. This transport was based on the B-2, and the six that were built served with Eastern Air Transport 1931-34. It could carry up to 18 passengers, a large number at the time, but this advantage was negated by its other, out-of-date, features.
The DC-1, essentially a prototype of the DC-2 first flew 1 July 1933 and the first flight of the DC-2 was 11 May 1934. The DC-2 soon became the dominant mid-1930s airliner.
The first flight of the DST was 17 December 1935 and it entered service with American Airlines the following summer. Knowing that its Condor IIs were obsolete, American Airlines pressed Douglas to create a sleeper version of the DC-2 to replace the its Condor II sleepers. A non-sleeper version of the DST was the famed DC-3, a few of which are still serving 75 years after its commercial introduction. The visual distinction between the DST and DC-3 is in those small, slit-like windows above the main windows; they were to allow upper-bunk passengers to peek at the outside world.