This sounds odd, but the US was in no hurry to develop land based bombers, as it had no one to bomb. Despite Billy Mitchell demonstrating that bombers could sink unarmed and immobile Dreadnought type battleships, the US Army did not diligently pursue any anti-ship capability. They understood strategic bombing, but any potential targets were too far away. Really, until the B-29, there was nothing the US could use from the US to Europe or Japan. The B-15 and B-19 were great acheivements but the goal was mainly to judge the state of the art and to figure out what was needed to build a truly strategic bomber.
The USAAC pursued the development of heavy land-based bombers throughout the period, and bought the Martin MB-1 & 2, various Keystones, the Curtiss B-2 Condor and more, and they had specific targets in mind, as they were fighting multiple wars to control former Spanish colonies for US corporations, none of which required great range or anti-ship capabilities as they supported army activities from nearby bases. Europe wasn't on the radar, but the distances were not unlike those of the transcontinental flights the airlines were achieving. Anti-ship capabilities were provided by a whole series of USN bombers from the end of WW1 and uninterrupted throughout the interwar period, including the USMC's well known Helldiver dive bombers and and torpedo bombers such as the Douglas T2D/P2D (a twin engine design), Great Lakes TG-1 and BG-1, Martin SC, TM, T3M etc.
The US never faced aerial opposition though, and they sneered at non-US developments, which resulted in their specifications becoming mired in fads disconnected to real world opposition, hence the shock of coming face to face with the Japanese, whose aircraft were almost all far better than anything the US had in service - which they attempted to write off with racist propaganda, about the Japanese aircraft somehow being both copies, and being more capable. At the same time, the Navy's torpedoes had never been tested in real world conditions either, and failed miserably - but they did have them,
The US military consistently lagged behind their own commercial developments, with airliners almost always beating their best fighters. Stressed skin, closed cockpits, cantilever monoplanes, retractable undercarriage and other drag reduction measures all became common in airline service long before being accepted by a VERY conservative military. The B-17 owes more to the 247 airliner as a four engine development, than it does to any prior bomber and the B-18 was a warmed over DC-2. Hence why almost all of the medium and heavy bombers that weren't completely obsolete in 1941 came from companies at the forefront of commercial aviation - Douglas (B-18), Boeing (B-17), and Lockheed (Hudson) being prime examples. This trend would continue until late in the war, when massively ramped up development spending meant the USAAF would finally beat the airlines, at fielding jet aircraft, but that leap forward came too late for the war effort.
The B-15 and B-19 were both dead end monstrosities that exemplified the US tendency to gigantism, and neither clarified USAAF needs in the slightest, beyond being obviously useless. Instead, the US had to learn from the British that the most important criteria for a heavy bomber is its efficiency in delivering the most bombs, in the shortest period of time, for the lowest cost - in fuel, aircraft and crews. Slow gargantuan airplanes just don't measure up, and the Sterling had the same problem - far too much airframe for the job. The Lancaster, and Halifax were the ones providing the real lessons, not the B-15 and 19, hence the startling similarity of the Liberator to both, as all of them had been designed from the beginning for efficiency. At the same time, aerodynamics was making massive leaps forward, so much higher aspect ratio wings, with laminar flow airfoils and smaller airframes with much better ratios between load and empty weight became the norm. The final lesson from the British was that loading up a bomber with lots of guns (which were far less effective than when installed in single-seat fighters) also reduced efficiency, but the US wouldn't make use of those lessons until well after WW2, beginning with stripping most of the guns out of the B-36s.
