The Wide Field Imager (WFI) instrument for ESA’s next large X-ray mission Athena is designed for imaging and spectroscopy over a large field of view, and high count rate observations up to and beyond 1 Crab source intensity. The other focal plane instrument, the cryogenic X-IFU camera, is designed for high spectral resolution imaging. Both cameras share alternately a mirror system based on silicon pore optics with a focal length of 12 m and unprecedented large effective area. The here described WFI instrument employs DEPFET active pixel sensors together with readout and control ASICs tailored for this project. The WFI DEPFETs are 450 μm thick, fully depleted, back-illuminated silicon active pixel sensors. They provide high quantum efficiency over the 0.2 keV to 15 keV range with state-of-the art spectral resolution and extremely high time resolution compared to previous generations of silicon detectors for Xray astronomy. The focal plane comprises a 'Large Detector Array' (LDA) with over 1 million pixels of 130 um · 130 um size, permitting oversampling of the PSF by a factor >2. The LDA spans a 40 amin · 40 amin large field of view and is complemented by a small 'Fast Detector' (FD) optimized for high count rate applications. In the present phase A of the project, the conceptual design of WFI is defined and the necessary development of technology is performed. Critical subsystems with respect to the development are the detector function and performance, the real-time capability of the on-board signal processing chain, and the ultra-thin large-area optical blocking filter, which has to withstand the acoustic noise loads during launch. Various prototype detectors of smaller size than the LDA detector have been developed and successfully tested for this purpose. The results determine the preliminary design of the detector system for the upcoming engineering model of the camera. With a breadboard of the signal processing chain, the necessary steps for signal correction and filtering are presently tested in particular to evaluate the time needed for this. The modular design of the breadboard is based on the Microsemi RTG4 FPGA as key component for the frame processor. An on-board data reduction is necessary because of the high frame rate and large number of pixels per frame. Otherwise, the signal rate would exceed the available telemetry rate to ground. Tests in acoustic noise facilities, complemented by vibration tests, shall prove that the 17 cm · 17 cm large and 0.18 μm thin optical blocking filter survives the satellite launch without a vacuum enclosure. The filter foil is supported by a mesh made of stainless steel and protected by a filter wheel design that minimizes the loads to the optical blocking filter inside the assembly. Furthermore, the overall design of the instrument and its various subsystems has been further developed. Tradeoffs have been performed to obtain an architecture of the instrument compliant with the requirements to the instrument, especially with respect to energy, time and spatial resolution, quantum efficiency, instrumental background as well as technical budgets like mass, volume, power consumption and radiator area. After completion of the current breadboarding phase, an engineering model of the WFI instrument will be developed and tested. According to the WFI model philosophy, a structural and thermal model as well as a qualification model will be developed and tested before the flight model shall be launched in 2031 to the Lagrangian point L2 in 1.5 million km distance from Earth. The mission lifetime is planned to be four years with a possible five-year extension.