To distinguish the second, third and fourth generation Boeing 737 from other twin jet aircraft with engines under the wings and a conventional tail look at the cockpit windows: these are the same as or similar to that of the Boeing 707 and 727. The older versions still have the two "eyebrow" windows above the normal cockpit windows.
The first generation Boeing 737 has long nacelles under the wings, from before the leading edge until beyond the trailing edge. For the rest it has the typical Boeing 707/727/737 nose with eyebrow windows, and a tall tail with hardly a dorsal fin.
If there is one iconic jet airliner, it is the Boeing 747 with its partial upper deck. The "hump" comes in different lengths, but not along the the whole fuselage, and this is what makes it different from the Airbus A380.
The Boeing 757 has a lot in common with the Boeing 767, but has a narrower fuselage for example resulting a different nose. The most likely you are going to mistake the 757 for a Tupolev 204 however. Look at the cockpit windows and winglets to keep them apart.
It may seem difficult to distinguish a Boeing 767 from a 777. However when you look at the tailcone and main landing gear it is easy: the tailcone of the 777 is squared off and the main landing gear has six wheels per leg. The main differences compared to the Airbus A300 and A330 are the cockpit windows and the curved up row of cabin windows on the rear fuselage of the Airbusses.
Essentially the Boeing 777 is a “blown-up” 767, which makes keeping them apart often tricky. However when you look at the tailcone and main landing gear it is easy: the tailcone of the 777 is squared off and the main landing gear has six wheels per leg.
While the Dreamliner looks similar to the 767, 777 and A350 it is still easily recognisable. Apart from the nose with only four cockpit windows instead of six, the curved up wing (with raked wingtips) and the backend of the vertical tail slightly pointing up are distinctive characteristics.
Boeing B-17 Flying Fortress
Famous bomber aircraft from the WW2 era that was built in large numbers. Typical for the B-17 are the tailwheel configuration with large, single wheel main gear. Another goed recognition point is the curved single fin.
Boeing B-29/B-50 & Tupolev Tu-4
This heavy bomber is best recognised by its typical round, stepless nose with cockpit windows all around. The Tu-4 was the reverse-engineered Soviet version.
Boeing B-47 Stratojet
Boeing designed America's first true jet bomber aircraft. Given the not-so-powerfull engines at that time, six were needed. These are located under the wings, four in pairs and two in separate pods. Also typical for the B-47 is the fighter like canopy housing the two pilots.