It was decided to focus on bombing Britain's industrial cities in daylight to begin with. The main focus of the bombing operations was against the city of London. The first major raid in this regard took place on 7 September. On 15 September, on a date known as the Battle of Britain Day, a large-scale raid was launched in daylight, but suffered significant loss for no lasting gain. Although there were a few large air battles fought in daylight later in the month and into October, the Luftwaffe switched its main effort to night attacks in order to reduce losses. This became official policy on 7 October. The air campaign soon got underway against London and other British cities. However, the Luftwaffe faced limitations. Its aircraft—Dornier Do 17, Junkers Ju 88, and Heinkel He 111s—were capable of carrying out strategic missions, but were incapable of doing greater damage because of bomb-load limitations. The Luftwaffe's decision in the interwar period to concentrate on medium bombers can be attributed to several reasons: Hitler did not intend or foresee a war with Britain in 1939; the OKL believed a medium bomber could carry out strategic missions just as well as a heavy bomber force; and Germany did not possess the resources or technical ability to produce four-engined bombers before the war.

Answer this question, if possible (if impossible, reply "unanswerable"): What kind of bombers was Germany not able to build?
four-engined bombers