In 1991, shortly after German reunification, Air Berlin became a German-owned company and shifted its core business to compete more directly with Lufthansa, eventually becoming one of Germany's biggest airlines.

[12][13] Plans were made to start long-haul flights on West Berlin-Brussels-Florida routes,[12][14] in cooperation with Air Florida (an agreement to that effect had been signed in February 1979).

[22] In 2001, Air Berlin and Hapag-Lloyd Flug became the first airlines in the world to have their Boeing 737-800s fitted with blended winglets, wingtip devices that are intended to improve fuel efficiency.

[23] Air Berlin introduced scheduled flights (which could be booked directly with the airline rather than via a tour operator) in 1997, initially linking a number of secondary German airports to Mallorca.

[24] In the same year, the airline expanded beyond holiday destinations as low-fare flights marketed as "City Shuttle" to London, Barcelona, Milan and Vienna started.

[13][24] Besides Berlin-Tegel, these routes were opened at six German airports (Dortmund, Düsseldorf, Hamburg, Münster/Osnabrück, Nuremberg, and Paderborn/Lippstadt)[25] that until then had not been served by one of the rising European low-cost carriers.

[27] It was suggested that the reason for the group to establish a UK-based PLC instead of a German-based AG was to avoid the need to have a supervisory board and employee representation as required by the German law of Mitbestimmung or co-determination.

[citation needed] On 7 July 2007, Air Berlin announced an order for 25 Boeing 787-8 Dreamliner long-haul aircraft, with further options and purchase rights.

[citation needed] On 21 August 2007, Air Berlin acquired a 49 percent shareholding in Swiss charter airline Belair, the remainder being owned by tour operator Hotelplan.

[40] At the end of March 2009, a strategic partnership agreement with TUI Travel was signed, with Air Berlin and its competitor TUIfly purchasing 19.9 percent of the other's shares.

[47] In preparation for joining the alliance, Air Berlin made codeshare agreements with Finnair and American Airlines starting with the 2010/2011 winter schedule.

[52] On 15 June 2011, Air Berlin and British Airways reached a codeshare agreement covering some flights within Europe, starting from 5 July 2011.

[67] On 18 December 2012 Air Berlin announced that topbonus, its frequent flyer program, would be sold to Etihad Airways; only a 30-percent minority share would be retained.

[70] On 7 January 2013 Air Berlin appointed Austrian Wolfgang Prock-Schauer, former Chief Strategy and Planning Officer, as the company's CEO, replacing Hartmut Mehdorn.

[74] In October 2014, the Luftfahrt-Bundesamt denied Air Berlin authorization to operate 34 routes as a codeshare with co-owner Etihad from the 2014/2015 winter schedule as they would contravene the bilateral traffic rights between Germany and the UAE.

All Boeing 737-800s were to be phased out by 2016 as Air Berlin plans to focus their short- and medium-haul fleet on the Airbus A320 family to cut costs.

[80][81] Some days earlier, the airline announced plans to add flights from Düsseldorf to Boston, Dallas/Fort Worth, San Francisco and Havana by spring 2016.

[83] On 30 December 2015, the administrative court in Braunschweig ruled in favour of the German civil aviation authority (the Luftfahrt-Bundesamt) and against Air Berlin regarding some of their codeshare operations with Etihad Airways.

The shared sale and advertising of 31 out of 83 routes which were marketed by both were declared illegal and ordered stopped by 15 January 2016 as they were not covered by the bilateral air-traffic agreement between Germany and the UAE.

[90] In December 2016, Air Berlin announced Stefan Pichler's departure after serving two years as CEO and replacement by former head of Germanwings, Thomas Winkelmann on 1 February.

[91] On 28 September 2016, Air Berlin announced The new airberlin, a restructuring project including the reduction of its destinations from around 140 to 70, the focus on the Berlin and Düsseldorf hubs and on the smaller bases in Stuttgart and Munich, the closure of six other bases, the targeting of business travellers, focus on domestic German flights and on flights to Italy, Scandinavia and eastern Europe, the expansion of its long-haul network, and the loss of up to 1,200 jobs.

[96] On 5 December 2016, Air Berlin announced plans to sell its entire 49-percent stake in its Austrian subsidiary Niki to its own minority owner, Etihad Airways.

[97][98] It was also announced that Niki will take over several routes to southern European, north African and Turkish leisure destinations from Air Berlin as part of the new joint-venture.

[104] On 9 October 2017, Air Berlin told its staff that it would cease all remaining operations under its own AB flight numbers due to its negative financial outlook and bankruptcy proceedings.

[106] On 27 October 2017, it was announced that a 'consortium' of maintenance, repair, and operations provider Nayak Aircraft Services GmbH & Co. KG and Berliner Zeitfracht Group would purchase airberlin Technik, keeping over 300 employees.

[96] The technical branch was a certified EASA Part-145 maintenance organization with approximately 1200 employees providing services to both Air Berlin group aircraft and customers throughout Europe.

[117] In October 2016, Air Berlin announced it would close the technical bases in Stuttgart, Frankfurt, Hamburg and Nuremberg while laying off 500 staff due to restructuring measures.

All short- and medium-haul aircraft began to feature business class in row 1 with expanded services including an empty middle seat.

[90] In contrast to European pure low-cost carriers, Air Berlin offered free in-flight snacks and drinks until September 2016.

On all Air Berlin routes with a flight time of 60 minutes or longer, gourmet meals were offered, which were, according to the airline, created by chefs at "Sansibar", a famous restaurant on the island of Sylt.